For Example: This is what the animation for series like Sailor Moon use to look like for simple tv shows episode
Even cash cow shows like Dragon Ball had quality animation such as this for episodes:
Now in comparison. We can expect episode quality for Dragon Ball episodes to look like this:
What has resulted in the quality for animation to go down the crapper. This isn't isolated to only Japenese animation as I've notice American animation has opted for the cheapest design style for characters even if it results in a lot of clones.
Most Helpful Guy
Well, I believe the 2015 Dragonball animation is a generally isolated event. It's pretty well known that it has had disappointing animation, and it's quite inexcusable. Toei animation just done-goofed on that. Nevertheless, it's the studio that greatly varies with different strengths and weaknesses. (Like Kyoto Animation has great visuals but storytelling is ehh. *Lookin' at you Phantom World* smh)
So yeah, it really depends on who is making the animation (aka anime) and how large of a budget they have.
Like for instance, Redline (2009) has insanely awesome animation and was handrawn. That has, in my opinion, the best animation I have ever seen. If you are going for even more modern, check out God Eater (2015). Despite its use of 3D effects (I despise 3D), it has pretty outstanding animation in my opinion. 3D stuff has gotten better over the years too though (like Ajin wasn't horrible despite it being all 3d). Boku dake ga Inai Machi (aka Erased) also had decent animation, not to mention a superb storyline.
Another thing to note is that animation hasn't necessarily fallen in quality on the whole, it's simply prevailing style has changed quite significantly. Deciding which one is better is very subjective. I personally like the look of more modern stuff, but others hate it. The use of 3d and computer aid in animation has also emerged more in later years as technology developed.
As for American animation, a lot of it has to do with marketing and the style. My guess is that they have the option to draw better, more visually appealing styles, but opt not too... mainly because generally their target demographic. A figure like stuff in Adventure time is very kid friendly. It creates a sense of extreme distancing from reality and lets the cartoon thrive as fantasy. It probably also has to do with budget constraints and what's cheaper.
So on the whole, what happened?
-- Animation style has changed drastically. While different, not necessarily bad.
-- It still is the same as it was back then in that some studios have great animation. Some are crap. Some anime just have crap animation from that.
-- It's not really a timeline of animation, it's what anime you pick as a sample.
Most Helpful Girl
It marketing. It whats in and hot right now. Yes, back then it was quality. But it has to do with competition and the ones in charge of the production within the production companies. As technology grows so does the industry's standards, and people expectations of that shows they get to see. You have a valid point there. But this is due to what they like. If you noticed with Sailor Moon from 1992 to Sailor Moon Crystal, most of it is 3D, more brighter, colorful at most. Same thing with Yu-Gi-Oh! Only Yuugi and his Grandfather other than Pharaoh Atem had Tri colored or multi-colored hair. Same with GX, and than 5Ds. Now it got even crazier with ZEAL or ZEXAL and even Arc-V!
More anime shows are also doing things 3D and multiple color hair. The same with American animation as well. Most quality shows like Batman, Batman Beyond, Recess, etc had moral themes to them, something that you can understand and relate to. How can I relate to a cartoon that basically tells kids its okay to hit your friend or somebody with a hammer when they tick you off?
It depends on who does the animations. In fact, animators don't get paid very well, especially in Japan. A lot of them won't even see a paycheck until the show/movie/video game is complete and it makes more money, than they will see that paycheck, but that don't get a lot of money.