LogFAQs > #976251625

Tyranthraxus posted...

Doc makes it explicitly clear that you can't go back to the nice future because Marty's first suggestion was to stop old Biff from getting the almanac in the first place. But how did Biff get back to it then?

It takes time before the future is changed, which is why Marty wasnt erased instantly when his mother never fell in love with his father.
So old Biff returned, and Doc and Marty went back in time just before the timeline they were in got erased. They returned to the changed 85 timeline because it had already been changed, and by the time they realize it the future timeline is changed as well.
So if old Biff had returned an hour later, he would have come to the changed future.
There is a cut scene showing old Biff being erased just after Doc and Marty going back to 85, but they cut it because they thought that it would confuse people too much.