What else do we really need to know about the adventures of Doc Brown and Marty McFly? We spoke to Travis Knight, who runs the animation company Laika [Boxtrolls], about sequels in general. His company has never made one, and he specifically said that "when you look at a story, ideally, the story should explore a pivotal moment in the protagonists' life. If we're doing a sequel, by virtue of what it is, it's going to be a diminishment. The second most pivotal moment of his life?"
This is as succinct a statement of my own philosophy on the matter as I'm ever likely to come across or utter on my own. A story has at its center a character, and the story is about something momentous in their lives. Everything after that tends to be, as far as drama and storytelling are concerned, "the second pressing of the grape" (as John Wayne once put it so floridly in his gloriously awful Genghis Khan pico-epic The Conqueror).
Sure, you can spin up more stories about that person — but that typically comes at the cost of trivializing everything else that you said about them. What was once drama, or even tragedy, devolves into soap opera, and whole soap opera is superficially entertaining, it's as ephemeral as the bubbles of the product for which it's named.
Part of why I come down as hard as I do on ephemeral and disposable entertainments is because we are creating a world in which nothing is ever really ephemeral or disposable anyway. The obscurest 1980s TV shows have fansites and fandoms; the most popular ones are getting multiple live-action movie reboots. Everything is preserved; nothing vanishes.
What's more, I wonder if the result of such an environment is a disincentive to create anything that reaches further and shoots higher, since one can safely count on some audience somewhere to find the meaning, purpose, and resonance that you didn't bother to put there yourself. Or never intended to.