Seems true

Demon slayer got consistency in animation quality.
There are plenty of animes that I like so much but it got shitty at some episodes or some scenes where things just went "wreck"
Example: Dororo ep 15.

black clover right now. They have great design but the fight scenes can be bad sometimes where I wonder why they even bothered. I wish they took their time.

Traditionally that was a defining trait of most “Japanese” style animation- individual cells were more detailed with characters themselves having more attention to small details like clothing, shading, etc- backgrounds would be between extremely detailed and literally so little detail they were splashes of color. Individual frames would be reused many tiles however, sometimes flipped, filtered, or slightly altered. That’s part of what gave the art its style and what defines how shows were directed.

On the whole “western” animation put more detail into backgrounds, and used the basis of anatomical models; but overall used a lower level of detail and higher number of unique frames. There was more actual “movement” in western animation as opposed to tricks to impart movement, recycled stills, or recycled movement frames.

Hana Barbara brought in a style of animation that was just plain cheap. It used even LESS detailed characters than western animation traditionally did, it had less detailed back grounds, and recycled as much as possible. Movement was kept to a minimum and tricks were used instead of actual animation. Characters were designed around this philosophy. That’s why “collars” and the like are so popular in that style- Yogi Bear for instance wears one. Why? So you don’t have to animate the movement of heads. You can’t see the break between head/neck/body so you just have to draw the face in perspective if you want the head to move.

Older “Japanese style” animation had some of the most beautiful shading you’d find in hand drawn art. Much was done by applying sheets of shade or pattern over a drawing and cutting with an exact knife, but much was also hand drawn. It’s an intensive process and involves thought and composition and often changing shade patterns when reusing frames or showing movement.

But animation is risky and expensive. Styles have changed over time to allow cheaper animation and reusing frames and animations; as well as the turning over of animation from a lead artist and a team to teams of “artists” who need characters that are simple and easy to reproduce in a consistent form.

Computer animation further cuts costs and allows for reusing sprites and frames and making templates and character sets generically. More complex characters not only take more memory to render and store (not a huge issue nowadays...) but they are more complex and dynamic. You can’t as easily just choose a pre made expression face and action from a character set and add a thing or two.

The “better” the sprite looks the more your brain will notice when little things are wrong. A photo realistic human makes it more obvious when the lighting is wrong or the wind should be moving things a certain way or clothing isn’t facing the right way. It also makes it harder to have enough generic frames to stack so you don’t notice consciously that they’re recycling.

So basically part of the change is stylistic for the times- but the primary factors are wanting to be competitive in the market. Most very young kids won’t notice the difference between painstaking art and mass produced marketing- and older kids or adults will notice- but outside of major releases they won’t care enough to make a serious difference in profits.

You’ll lose a lot more money if your animated series isn’t a hit and is beautiful than you’ll gain if it is a hit and is beautiful over if it is a hit and is poor quality. It’s hard to sell a project for an unknown franchise based on space wizards or a child and their robot or talking sea creatures when you’re asking for millions and some other guy has a sea creature cartoon he can do for a hundred grand. The huge success of poorly drawn but well written or engaging animation built a model that you can create huge and profits with cheap animation.

If you make 20 “cheap” cartoons and 19 fail but just 1 is Invader Zim or South Park- you’ve paid back all 20, plus a huge profit, plus financing for 20 new tries. If you make one Miyazaki epic and it flops- you’re done. If it takes off- you have profit and enough money to make one or two more. You’ve got way more chances of finding a hit in 20 tries than 2. So I think that while there is SOME discussion of style to it- more than anything it’s about money.