The main issue is that the roles I mentioned before
are the roles the various units are intended to fullfill. There is then still some slight specialisation with load-outs (e.g. a bastiladon with solar engine vs. ark of sotek). But the roles I mentioned are their basic battlefieldroles. However, as I said before, the game does a poor job of distinguishing these roles. There are a couple of reasons for that:
1) We're balanced around points. Which leads to the "odd" situation where equal sized forces where one
should have a distinct advantage over the other frequently
don't. As an example; a 1000 points of heavy infantry should absolutly decimate a 1000 points of light infantry without much effort in a straight up brawl as heavy infantry is just flat out better (with a few exceptions like light infantry armed with specialized anti armour weaponry or some such). However, not only doesn't this necesarly happen, there's examples of light infantry trouncing heavy infantry.
2) The differences between the various roles are rather minor in AoS. We lack things like a strength/thoughness mechanic, to distinguish between heavily and lightly armoured troops. Wounds carrying over means fodder can't absorb strong attacks. And the differences in stats, especially defensivly usually aren't particularly major, which combined with the approach to balance then results in the situation where a squad of 5 heavier infantry costs a 100 points with 2 wounds each and a 5+ save, but 10 light fodder costs 90 points with 1 wound each and a 6+ save, making them relativly comperable in terms of effective wounds per point (difference is about 1.66 effective wound per 100 points). So unless you bring say 2000 points worth of each the difference can be negated by a (un)lucky roll or two. In fact right now the only mechanic that properly differentiates between lighter and heavier troops is battleshock as cannonfodder is much more likely to suffer from battleshock and to lose more models at once and often has worse bravery as well, however command abilities that prevent battleshock are stupidly common, effecitvly neutering this mechanic as well.
3) GW doesn't necesarly give models rules (and stats) that fit with their intended role. Which leads to weird stuff like judicators having an anti-horde weapon that works consistently as long as the "horde" consists of 5+ models and only really becomes unreliable once you drop below 3 models. Or a melee champion-style hero like the lord celestant having powerfull ranged attacks for some reason. Which further muddies the waters, as well as ends up breaking what should be the counter to this particular unit (a lord celestant shouldn't be winning a ranged firefight, even if it is against the worst archers in the world, and judicators shouldn't be decimating a small elite heavy infantry with rend immunity unit like saurus guard)
As for your remark about not needing multiple units of the same role. You do, at least in a few instances, obviously with some differences like a focus on anti-horde vs. some extra survivability or some such. It adds flavor and variety to armies. Otherwise you'l end up seeing more or less the same stuff constantly. Simply sticking with different loadouts will probably not be enough as then people will probably end up sticking to whatever is most effective in the current meta (just look at all these skinks with clubs that don't exist cuz skinks with clubs are terrible)
Anyways, long story short, the solution shouldn't be to just dump half of the unit roster but instead to actually make meaningfull distinctions between the various roles.
As for the roles you mentioned, those are what you get when you look at it from an extremely game-focused point of view and ignore that the game ultimatly is a wargame and is supposed have some kind of basis in the reality of war. In itself that is of course a valid way of looking at it. But it does mean you largely lose the war aspect and that indeed with 5-6 units you have a functional "army". However it will mean that every army from a given faction will play basicly the same, as well as losing the war and the collectors aspect of the game. It'l probably also limit the amount of variety between armies to a significant degree as well (after all, if you're the only faction with cavalry we can't just make it twice as fast as infantry as you'l just be running circles around your opponents)
