[jkarneges]

That's true. BTTF isn't normally predeterministic [1] though, but maybe the desired result could be achieved through a couple of history rewrites. For example, young Marty visits a weird future where old Marty doesn't exist, then he goes back to the original present to live normally. We can then envision a replay of young Marty's experience in the future, wherein things fade in/out around him to create a world where old Marty exists too.

[1] By predeterministic I'm thinking of something like Crichton's Timeline, where the characters visit the past not to change it but to fulfil it (i.e. there is no rewriting or branching, their existence in the past is a constant).