If you apply that logic, then Tomb Kings would serve pretty much every Fantasy faction except for Beastmen and Bretonnia. Are you willing to accept that?
Luckily I don't have to accept that as you've simply presented a false equivalency. Necrons (and other 40k Xenos) are left in the shadows as GW almost exclusively pushes their Primaris line. The Tomb Kings did not share this same fate under 8th edition WFB. They received a new book and an absolutely fantastic line of new models. All 8th edition WFB armies were roughly in the same boat (with the exception of Skaven, Bretonnia and Beastmen). There is no Primaris equivalency in 8th edition fantasy, nobody got the lion's share of attention and love. Your newfangled Necrons are eternally cursed to squeak out a pathetic existence in the ever looming shadow of the Primaris. Now, TK in AoS is a completely different story... but that has absolutely zero bearing on me. Actually, the fact that the TK are discontinued in AoS only makes them that much more valuable and precious to me.
Sure the Tomb Kings book is a bit weaker, but that is down to them getting a book so early in the 8th edition cycle (2nd, after Orcs). They are still a viable army, but require a bit more skill to wield properly. Besides, I have my powerhouse WoC to fall back on should the need arise. I'm actually happy I have a good spread of power levels between my armies (WoC, LM, CD, TK).
subtle but intelligent (Prequels and Szarekh)
This is a perfect example of how to lose a debate with a single poorly thought out idea. By claiming the Prequels to be subtle but intelligent and then linking them to your new Necrons you've completely destroyed your own argument.
Here is your "subtle but intelligent"...
The prequels are anything but "subtle but intelligent". In fact, I wouldn't assign those descriptors to any of the SW trilogies. In any event, I'm not here to discuss the OT versus the PT with you on this thread. We've already been down that road many times before and history is not on your side. I merely mentioned the PT versus OT comment in my previous post to highlight that we always fall on the opposite side of things.
I’m not saying that being god of death itself is negative, I’m just saying it’s a samey trope, especially when you have Nagash being the exact same thing in AoS.
It's funny how you can say that and then in the very same breath you can exclaim that the new Necrons are Space Tomb Kings (and likening Szarekh to 40k Settra)! The TK themselves are based on a long existing trope. Either you're against the "samey trope" or not. I don't see any consistency here on your part.
But when Szarekh returns, he will unify the Necrons into that far more powerful threat.
When Szarekh returns he'll probably be a Primaris!
And as for being a more powerful threat, probably not as much of a threat as a Necron army that was unified from the start under the rule of Star Gods.
Besides, I’m pretty sure even in the 3rd Edition era there were stories of the Necrons having their backsides kicked by Space Marines.
It was starting to get bad, but absolutely nowhere near the level it is today. You didn't have a billion armies based on various SM factions.
If you want an account of a Space Marine - Nightbringer interaction, in the Black Library novel form, might I suggest...
Or the new cover if you prefer...
Szarekh is kinda bland... that's why the Silent King is so much better and impressive!
I can't argue with that. Although, as names go... Nightbringer > Silent King > Szarekh
) and the Silent King on the top right (fantastic miniature, but most likely over £50 in price 
