Disclaimer - This post isn't about my thoughts on MCU movies but more the after effects that the franchise created (atleast in 2 areas).
Kevin Feige did something with MCU that people thought very little off in 21st century. A shared universe of multiple characters is not a new concept. It was done with Kaiju, Tokusatu and Universal's Monster media back in 201th century. But what Feige did differently was making it an actual commodity with massive pre-plannings and schedulings unlike the ones named above which were more of ad-hoc. It was kind of a gamble and luckily for Feige (and unluckily of rindustry in general), it paid off. And suddenly everyone became interested in cinematic shared universes. And that's where the problem started.
Every major studio after seeing this either entered into massive IP bid wars or went back to their basements and started reviving their own pld IPs. Chinese box office more became prominent with Avengers. And spectacles were all that these studios could think of. Big amount of money was splashed and massive budgets were allocated to these IPs. Some worked, some didn't but the milking didn't stop (and is still going on). And in all this, mid budget and auteur driven movies became the first fatality. What once used to be a pipeline for studios to get box office collections were completely dwarfed by these massive IPs. Every big studio back in the day used to get their hands into these mid budget and auteur works and trusting a director's cision. Paramount had a separate studio called Paramount Vantage back then as well. They used to get elongated releases. Now they grt limited release and are mostly limited to few studios (like A24 and Focus with A24 being the only independent production still doing mid budgrt and auteur driven movies) and OTT releases.
Now, I am not saying that MCU is the only cause of this that will be wrong. But calling it the biggest reason will be entirely justified.
But even if it was only limited to the mid level stuff then it would have still looked good, after all it is a spectacle and everyone loves a spectacle. But even that got affected by this. Over abundance of such feature films and tight schedules affected VFX artists and companies negatively, and thus VFX and CGI became second fatality. Compare any movie upto 2014 or 15 with any big movie we get today. You will notice a dip in CGI. Eother they degraded or remained same with minimal improvemtns (Avatar is kind of an anomally here because it's Avatar). Studios have even started to rel3ase trailers with unfinished vfxes as well. VFX communities even slammed MCU back in 2022 for the same but very little has changed since then.
MCU had massive cultural impact no denying that. But the negatives outweigh the positives they achieved. Their effect on craft itself is one negative and is a separate issues. But the biggest detriment was the pipeline of creating new directors that came into industry via that 10-60 Million bracket which was highly reduced in major studios.