"Avatar" nailed "alien planet". "Harry Potter" nailed "magic". "Titanic" nailed "big disaster". "Lord of the Rings" nailed "fantasy epic war". "Batman" nailed "comic book hero". The backlog has been worked down. Audiences can no longer be impressed by doing any of those things.I'm very suspicious of the argument that all the stuff we can imagine has been done (for one thing, there's a world of sui generis stories to be told about subjects that can only be approached with CGI -- also, there's probably not one canonical way of visually representing subjects like "magic"), but I really hear you about the cost structure of Hollywood driving an inherent conservatism in subjects and approaches.
It wasn't cheap. Movies once boasted "a cast of thousands". Now, major films do have a cast of thousands - of artists and animators. "Captain America"'s credits have about 850 people on the effects side alone. Anything can be put on screen, but it costs about $100 million.
That's the problem. The technology didn't make movies cheaper to make. Even if the whole thing is done in front of a green screen, it doesn't save much money. ("Sky Captain" was supposed to cost $20 million, but ended up costing $80 million.) We're not seeing good $20 million movies with high production values. Those economics lock Hollywood into what are considered sure wins.