I mean off the top of my head, Black Clover has ~170 episodes and that came out late 2017 (4 years). The “seasons” are broken up into 40-60 episodes, that’s a lot.

I don’t think it’s the end of long form content, just not many adaptations benefit from continually running or have that amount of content that they could do weekly releases for that long. Your not going to have a romance anime go for 300 episodes…

Most anime that do this long format are shonen where you can throw filler in about the tournament arc while waiting for the Mangaka to catch up.

There’s also a quality argument, I’m guessing Demon Slayer is going to hit around 100 episodes by the end of the series. It could’ve done the long form series, but I don’t think it would’ve gotten the same attention or quality that it has received. It’s also safer to start a show for 12-24 episodes and then see if it’ll flop, then throw a bunch of money at an unsuccessful show.