A single wizard with a bonk stick simply should not out fight 10 clan rats.
Similarly, what is a starpriest supposed to when fighting 4 clanrats?
I said 4 rats; not 10. And imho, any hero should be able to defeat a MSU of basic troops that has been reduced to 20% strength.
However, that's not even the important thing. The important thing is that the starpriest; a unit who has put 99% of his points into magic; only has heavily specialized spells, and has no (halfway decent) generic tools in its toolbox (spells or otherwise), so it just ends up standing there being useless even against some of the weakest enemies in the game. Even if you don't think the starpriest should win against this particular amount of rats; it should at least be able to reliably do something. But with its lack of useful spells; and abysmal attack stats, there's a reasonable chance it won't even manage to land a single hit over multiple turns. Which is silly.
I'm not saying it needs a massive buff; all I'm saying is give it some generic tools so it can at least do
something. Even a single point of reliable damage per turn, against something as minor as a handfull of clanrats, would already be enough
.
Also, giving a unit -1 rend, which it can, is kinda, almost giving you +1 save, with the bonus that it's an unlimited spell so all your wizards can cast it.
Spoken like a true competitive player

Anyways, yes in theory it's equivalent to a +1 save. Which is great and all. Until you face an army that doesn't rely on rend.
Which is kind of the problem with all of these variants of minor buffs that give a +1/-1 to a stat. In theory they should all be equivalent. But that's only really true in a vacuum when looking at a generic 4+/4+/1/1 style attack. In practise; when interacting with other rules/warscrols/stats, they can wildly fluctuate in terms of utility depending on the specific scenario you happen to be in.
-1 rend is great; until the opponent has no rend.
-1 rend is great; until an opponent has too much rend (e.g. 2 rend and you have a 5+ save)
-1 to wound is great until an opponent has crit (auto-wound)
-1 to hit is great, until an opponent has a baseline 6+ hit and is relying on nat 6's anyway
Anti-infantry is great; except you already have 1 rend and your opponent only has a 5+ save.
etc.
Which is fine if your unit offers more than that 1 single buff. But wizards often end up in situations where that single buff is their only contribution for a whole turn. And if your sole contribution is useless forcing you to stand around for a turn doing nothing then that is lame.
Why need universal spells when they've given you better universal options with manifestations and opening up the spell lore?
Because the spells revealed so far are all heavily specialized; and over-specialization results in scenarios where your wizard is a useless paperweight.
This is the most visible in the manifestation lores where they've arbitrarly cut things up into little groupings.
I get seperating the Krondspine from the others; but the others really didn't need to be cut up into 5 seperate groups. And especially lores like "forbidden power" which for some reason has 0 offensive spell, but it does have 2 teleports, are just overspecialized.
If they had at least said you get 1 generic lore + your faction manifistations as endless spells; this might've been acceptable. But we don't even get that (plus; it'd still screw over the factions without their own manifistations but at least that can be fixed with new releases

)
And of course, it is possible that the manifestations & spell lore end up having decent generic tools in their kit. Maybe each lore is going to get 10 spells, ensuring there are enough spells in the lore to give something usefull in every scenario. But that seems unlikely.
Also for the record; having manifestations be "free" and giving the entire spell lore is an improvement over 3th.
But removing arcane bolt and mystic shield is step back.
Also, the removal of nearly all warscroll spells on units shown so far doesn't help either.
Basicly; 4th seems to be 1 step forward, 2 steps back in terms of magic.
Also id be willing to bet a smaller gap exists between competitive and casual players than you think. I play pretty competitively in some circles, but casual on complete others. I don't find the way you talk about the divide to be very reflective of my own personal experience.
Ironically every competitive player I've ever met; in every single game, is convinced they are totally down with the casuals.
It's funny to see how insistent some tournament going players can be that they are totally casual. Seriously, I've seen people claim to be casual because they never won a "big" tournament and "only" win small local tournaments. It's hilarious

.
Anyways; the reason you won't really notice it, even if you play with a lot of casuals, is because most casuals won't really put this much thought into things, and won't be able to put it into words. If they are even consciously aware of it. They're more likely to just stop using their starpriest when it's been useless once too often. Or possibly just quit the game entirely. Or just be perpetually mildly dissapointed in their starpriest.
And occasionally, they're lucky enough to largely avoid the problems. For example, if their prefered playstyle happens to naturally fit around the potential problems.
It requires a rather specific type of player to both notice and care about this kinda stuff.
Completely off topic, but with the rework to priests, I kinda wish the Starpriest was, well, an actual priest again. Oh well.
More on topic, I can absolutely see both sides of the argument. As a more "causal" AoS player but a "competitive" enthusiast, I absolutely see the value in taking the most optimal spell lores but concur with having casters feel useless. That happened a bunch with my Starseer, he'd be a little too far away to use any of his set spells, so Arcane Bolt was helpful to have in a pinch. One thing that I've noticed though is that several units seemed "good" or "bad" in theory on paper, but during play actually performed the complete opposite.
For example, as a Coalesced player who didn't field a Slann, the Starseer was a must-include due to the 2 casts, the 5+ ward ability, and Speed of Huanchi. In practice, I continually failed half of the rolls to apply the 5+ ward and rarely succeeded or needed the Speed of Huanchi, leading to basically a wizard not worth its point cost.
On the other hand, I've seen multiple people state how fantastic their Bastiladons with Solar Engines performed, despite their average damage output being somewhat underwhelming.
I'm not really sure what my point is, but I'm at least highly impatient for the full Index release which is presumably next week. I really can't wait to start list building and seeing what units are potentially more viable in AoS 4 than 3. If anything, the Slann reveal has convinced me to buy and paint one up, same as the Kroxigors.
This is part of what I keep talking about; things like baseline performance & having the generic ability to do
something are very important for how good a unit feels. A unit that reliably does something mediocre will often feel better than a unit that occasionally does something great, but is frequently just sitting around doing nothing. Even if on paper that 2nd unit is "better".
Solarengines don't have particularly great damage output, but they pretty reliably hit at least 1 shot per turn. Which makes them feel good, cuz every turn they did something.
On the other hand, that starseer only gets to to apply his ward-save once per game. Fumbling that 1 attempt due to a bad dice roll feels bad, obviously. But even if it works, it's still only once per game. The other 4 turns your starseer needs to find something else to do. The same can be said for speed of Huanchi; sure occasionally it's very useful, but in between failed casts, and turns where you simply didn't need it, there's going to be a lot of turns where again; your starseer isn't really doing anything. And not doing stuff feels bad.
Competitive minded players often don't mind; they tend to focus on that 1 turn the Starseer did something extremely useful.
More casual minded players instead tend to focus on the fact that the solar engine was at least a little bit usefull every turn while the starseer spend a lot of time doing absolutly nothing.
O, and the further you need to look to see the "value" of a unit the worse this effects gets. A unit that is useful every turn feels better than a unit that's only useful once per game, which feels better than a unit that is only useful once every 10 games. Even if that last unit is the single best unit in the game and wins all the tournaments, it will feel like a terrible unit.