system
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human
[Back to the Future Part 2] When Marty went to 2015, shouldn't he have arrived at a timeline where he mysteriously vanished in 1985?

It's not as though he was in Hill Valley again until 2015 once he got in the Delorean, same with Jennifer. Those two and Doc should have wound up in a timeline where they just went missing, yes? Did they just visit a parallel universe?
gpt
The Delorean travels through time faster than the ripple effect of time changing.  This explains why Old Biff came back to the 2015 we already knew, but then disappeared as past events within the timeline caught up to him (deleted scene).

When Marty travels from 1985 to 2015 we are seeing the most likely future. This would also explain why Doc is in such a hurry to get his plan done and save Marty Jr., he knows that the ripple effect will catch up to them if they don't return to 1985 fast.

This also explains why Doc is in such a rush to get Marty to the future at the end of Part 1.  You'd think, they have a time machine, what does it matter if they go and fix the future now or next week? Well, since Marty just changed his family's history during the events of Part 1 there is a ripple effect catching up to him.  Marty still remembers the old life (we see his reaction to his rich good looking parents after he wakes up), but in time the ripple effect will overwrite these.  And perhaps in this new timeline Marty never had any reason to seek out and work for Doc.  Doc needs to get to Marty before their history from the original timeline is erased, so Marty still remembers him.