Honestly, I think with streaming services and the continued internationalization of the film and TV business, over time these conventions of what's expected in terms of the length of a movie, the length of a TV episode is going to change.

That said, it is interesting how movies evolved from single-reel films to double-reel films and so on to eventually land on two hours as what has been the conventional length of a movie. Films have succeeded with that length (more or less) for decades. Perhaps there is some psychological thing at work where we need that much time to feel like we've seen something of substance, but don't need more than that to tell a cinematic story.