I am making this post because many of youmisunderstand BTTF time travel.

BTTF has a single, non-branching, mutable timeline. Changes to the timeline propagate to the future slowly, at some sort of undefined meta-time rate. This is why Marty and his siblings start disappearing from the photo one by one (although why they do so in parts is nonsensical). If it used branching or multiple timelines, the photo would be unchanged, just from a different timeline. If the timeline was immutable, there'd be no fading problems, no mall sign change, no change in his parents' lives, etc.

A single, mutable timeline with non-instant change propagation is the only way to explain those films. It's nonsense in reality, but it's pretty consistent in the films.