I've always been a little mad about it because 'make big shield to protect people' isn't a tank thing in the game's cutscenes, that's a healer thing. Y'shtola used to do it a lot!
The game's always been in need of 'cool healer moments', where the healer feels like the big, center-stage hero. Even the actual healer-focused mechanics are more often the healer doing smaller complex stuff like having to clear Doom; 'make a big damn shield' is exactly healer's brand for a hero moment, and yet... well, it's a tank mechanic! (Even if other people can do it, it's intended as a tank mechanic, the healers have healing to do, the tanks don't ahve tanking to do.)
Dikatis did a good job showing that your read on all this was pretty narrow, but to add to it:
The thing with the people who had to be put to death for tempering is that they were fantatical and violent. A cure would've been nice, everyone theoretically wanted that, but it wasn't available for a long time. Meanwhile, the people forced to summon Necron weren't really doing anything; they were sort of laser-focused on the summoning and just sorta vegetative otherwise. Ironically, they were probably more dangerous before the tempering, that one guy mugged someone for their neo-regulator. All this makes sense not only when you remember that Necron isn't exactly megalomaniacal, but also when you remember that Calyx is a very technically-minded person who just wanted them to do one thing; of course they're not exactly doing anything else.
The writers haven't lost their grip on their worldbuilding', and they're not disrespectful of it; I think you're overly rigid in your understanding of it. It's not that the thing can't be done: it's that you decided it can't, probably quite a while ago. When the truth is this all completely scans, this is essentially an echo of Ga Bu as Dikatis said, but also Venat as RedLolly said.
Bringing up ARR Summoner is a little weird to me, because it's kinda... neutral to the point. Tristan isn't proving or disproving anything about this, he's merely a consequence of a previous status quo. But I kinda suspect you're putting him forward as an unconscious appeal to 'old and more niche info is better': that because he's from an old storyline that other people might not have touched or remembered, he is therefore 'greater evidence' than much clearer things like, again, Ga Bu and Venat.