What do you mean by that?
I'm not familiar with Doctor Zhivago so I had to look it up, the 1965 film is based on a 1957 novel, which seems to match just fine with the pattern of The Martian, The Three Body Problem, Game of Thrones, Fifty Shades of Grey, etc.
Back to the Future is a fun trilogy, but the first film was mostly set in 1955 (so if it had been done today the past would have been 1994, compare to Captain Marvel being mostly set in 1995), the second film was 1985's idea of 2015 (and we still get plenty of what-if fiction in the near-ish future, e.g. Blade Runner 2049, Cyberpunk 2077), the third was yet another wild-west Cowboys-and-Indians gunslinger (1885) — but if you mean to focus on the more playful and fun aspects of how it was put together, or the positive vibes of good people working to solve problems and where it's clear who the goodies and baddies are, BttF reminds me more of The Orville or Lower Decks. (And The Martian if you want to consider the environment/time travel itself, rather than Biff, to be the problem).
For that issue, I think that as any medium ages, it transforms from "explore" to "exploit", which is also why films have a lot more sequels and reboots.
So, if you want more range, explore new media: games, youtube, etc.
I've never agreed with this argument, it just felt like an excuse. For 100 years movies kept changing as society also changed, and all of a sudden they hit the exploit phase somehow after 2000, when GCI was still improving.
Movies are in part a product of a social climate, and that climate is never static, so to me it seems like there should always be something to explore. It just seems that financier no longer want to put any money in exploring, and culture becomes poorer while the GDP grows.
Even BttF itself was three films, and in the second one was parodying the tendency towards sequels with the holographic Jaws 19.
Of course, the world if large and kaleidoscopic. But the rate of homogeneity in the middle 80% is higher than it's even been, and that really affects everyone. I actually want diversity in the mainstream, since some projects are only enjoyable if you can share them with others and if they get the most talented people to work on them.
The fact that nobody would make a Doctor Zhivago or even a Back to the Future today is not something I can fix with all my searching.