That’s a situation largely of Netflix’s own making. There was a time when they were the only game in town for streaming movies and TV shows. They had the opportunity to build a gargantuan and loyal subscriber base and they blew it. Their decline is what opened the door to all this competition.

There's competition from outside of Netflix too. Plenty of other streaming services pump out content here too.

You might be happy with all this competition but then why are we even here for this discussion? It’s clear that too much competition has led to most shows dying prematurely.

Shows used to get cancelled all the time. Including long-form TV shows.

Star Trek TNG’s show runners knew how to deal with this: you don’t use the same budget for every episode. Some episodes get a big budget, others get a small one. The latter were called bottle shows and they turned out to be some of the best episodes in Star Trek history because they had no effects budget so they had to rely entirely on writing, acting, and editing.

Even the specific Star Trek episodes with the highest budget wouldn't come anywhere near to the usual per-episode budget of TV shows now.

But that's only one reason why the old 20-24 episode formula has declined. Not the only reason. Actors in TV now are also regularly in movies and many other shows. Many actors now worth hiring simply do not want to tie themselves down to 20 episodes a season, and define themselves by one show.