Inevitable Job Simplifications?

[Nizzi]

What does the vaunted bastion of high skill ceiling BLM get from 80 to 90? Well, Amplifier to give them a Polyglot stack, and Paradox which.......in practice just replaces a Fire/Ice when you swap elements. Oh, and they get a flashier Fire and Blizzard effect...uh...for AOE. Though to be fair to BLM, its rotation isn't in a more or less complete state until level 76, I guess? At level 70 you have Foul, which is what you replace by swapping it out for Xenoglossy at level 80 (so in effect, the rotation doesn't change, you just swap the button in that spot on your hotbar for single target fights). Once you have Umbral Soul at 76 (for downtime or recovery), you more or less have your finalized rotation, just you swap a button (Xenoglossy for Foul as your movement tool you try not to overcap resource on) and Paradox...actually takes the place of Fire/Blizzard 1 on your bar, so you actually don't even change the place your finger/thumb pushes on your hotbar. Not counting downtime/recovery, BLM's rotation is essentially complete at level 70 as well - though given BLM is BLM, I'm going with 76 on that one, but opinions may vary.

Paradox doesn't "just replaces a Fire/Ice when you swap elements" because BLM is not using Blizzard I to begin with, before level 90 you are using a Xenoglossy or Thunder III there along with Blizzard IV so you can get two MP ticks during Umbral to get back to full MP. For all intents and purposes it's an entirely new button during your Umbral phase that replaces a filler Xeno/T3 allowing you to move them to other places where they're more needed.
You say "rotation doesn't change, you just swap the button" for Xenoglossy/Foul but Foul has a cast time until level 80, meaning it is never used at any point before as a movement tool, so yes, the usage of your Polyglots pre level 80 and 80+ is different and does change your rotation as there's now more considerations on where you place Polyglots rotationally with the addition of having 2, the ability to use them for movement or to weave oGCDs, and with it being your most powerful ST spell.

So no, BLM is not "essentially complete" at level 70, the job develops and meaningfully matures in such a way that what you have to do to perform on the job is very different at level 70 than it is at level 90; same with level 80 to level 90 as well.

Summoner is not so. Once you get Emerald Carbuncle at level 26, the job fails entirely to develop meaningfully and give you any different considerations as you go from level 26 to level 90. Calling it "braindead" is an apt choice of words, because it is. It has no considerations that the other jobs within its role has; it does not have to think about positioning or casting at all, and it does not have to think about how it uses its abilities in a meaningful way at all.
Holding Aetherflow for more Festers during the 2m burst is not depth. Using Garuda first so you can get Slipstream out if you have a buff heavy comp is not depth. Could Summoner become a job with depth? Sure, but it is not so now. As it stands, it is horrible for the caster role and is an active detriment by virtue of existing. There is no reason it should be doing similar or higher DPS to RDM when RDM has to actively work much harder in the current meta to get the most out of their job with how fight design has turned out this expansion, and SMN is literally just "LOL I SPAM 1 BUTTON AND WIN".

It's fine to like the new SMN, but call it what it is. New Summoner fans like it because it requires no effort and gives them similar or better results to jobs that actually have to put in effort.