This is actually a pretty ahistoric take, so a few corrections and elaborations.
Calarts was founded by Disney, it was always intended to train animators for the industry and taught a very specific style and very specific techniques in order to train animators for their (presumed) jobs at Disney. That has always been the case with the school, but because it’s the school that Disney founded it acquired a strong rep, and because they are (or possibly were? I’ve heard mixed things) highly selective, and because their graduates would basically be guaranteed jobs upon graduation, it attracted a slew of artists who would go on to studios other than Disney.
I feel like what makes calarts and the calarts so much worse now has to do with the fact that almost no shows on the air come from people who aren’t calarts grads. Like you mentioned the 90′s and among the biggest shows to come out of that decade were Ren and Stimpy, Rugrats, Hey Arnold, and The Simpsons, none of whom’s creators went to calarts. In fact, while I can’t find information on where Arlene Klasky, Gábor Csupó and Paul Germain graduated from, among the other three creators only one of them, Ren and Stimpy creator John Kricfalusi, went to an art school with a focus on animation.
That meant that when Tartakovsky, McCracken, and Gutierrez were working, they were competing with shows that came from people who did not go to the same school, who had completely different art styles and had completely different backgrounds. I’ve also heard that it’s not just show runners who are all from calarts; the typical crew working in any given well known studio also tend to be from calarts more often than not. This means that not only do showrunners come from the same school with the same training, so do most other artists and writers contributing to their show. I can’t say for certain, but it’s very likely Tartakovsky, McCracken, and Gutierrez had far more people working on their shows who did not come from calarts than their modern counterparts, which also meant getting different input and influences from different schools.
The problem isn’t just the calarts style or that they train their artists to be employees first and artists second, it’s that animation as an industry has become so insilar that there’s very little outside influence
(As a sidenote; It’s actually kind of funny that you brought up Rebecca Sugar because, while many many calarts grads imitate her style, she’s one of the few animators out there who created a show who did not graduate from calarts.)
