According to the rules set up by the film's own internal logic, as soon as old Biff is successful in going back to give young Biff the almanac, shouldn't the environment around Marty and Doc immediately change?
The simple solution would have been for things to change around Doc and Marty but they just don't notice (or maybe Marty has a line... 'was that tree always there?'). That sort of thing. Job done.
We love discussing this point to DEATH on these boards!
Here's my take: In the BTTF universe, changes to the timeline manifest gradually, maybe because the "future" as we see it is at least possible.
If, for example, Marty had accidentally KILLED George, perhaps he himself would immediately vanish.
Instead, Marty only interrupts his parents' first meeting. Because it's still POSSIBLE that Lorraine and George will meet and produce his family, Marty and his siblings only START disappearing from the photo as their existence gradually becomes less and less probable.
Similarly, Biff's high living in "Hell Valley" likely killed him, so old Biff in 2015 was feeling the pain of mortality catching up to him. And in the deleted scene, of course, we see him actually fade away.
In contrast, at the end of this film, the matchbook and newspaper clippings change immediately once Gray's Sports Almanac is actually burning. But until the book's on fire, it's still possible that Biff could get it back and create "Hell Valley."
I'm speculating too much, I know, but the point is, changes take time to manifest.
But THAT would mean they should arrive back to a normal 1985 and then things gradually start to change around them.
shareMaybe the timeline is correcting itself step by step, so the further you are away, the longer it takes until the time process to overwrite everything is done. The changes in 1 took potentially longer, because the event really changing the history has not yet happened. In 2 the changes happened before they did the time jump.
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