[Laserschwert]

Originally Posted by Billy

Another thing that bothers me is the fact that when Marty and Jennifer goes to 2015 at the begining of the 2nd movie, they arrive in a future in which they are still there, 30 years older.
They should have gone to a future in which they mysteriously disapeared in 1985, the morning they were supposed to go to the lake, and were never seen again.

Think about it. It really doesn't make sense otherwise.

Taken from the trilogy's bonus DVD FAQ:

Q: At the beginning of Part II, when Doc takes Marty and Jennifer out of 1985 and takes them to the future, how can Old Marty and Old Jennifer (and their family) even be in the future? Wouldn't their disappearance from 1985 instantaneously erase their future?

A: To be honest, yes, it very well should erase their existence from the future. This is, in fact, the ultimate paradox of BTTF Part II. We really thought about this one for a long time, but we finally decided that after the setup of Doc's saying "something's gotta be done about your kids," the audience would feel cheated if we went to the future and found out they didn't exist.

You could, however, argue that the existence of Old Marty, Old Jennifer and their kids in the future automatically proves that Young Marty and Jennifer will eventually get back to 1985. The flaw in this reasoning is that Doc repeatedly tells us that the future isn't written, so why would this part of the future be "written"?

Ah, but Part III may contain the answer to this question after all. When Doc spots the tombstone in 1885 and sees that the name on the photograph of the tombstone has vanished but the date remains, he says "We know this photograph represents what will happen if the events of today continue to run their course into tomorrow." That's a pretty big "if", and it suggests that time travel to the future always takes you to a future based on the events of the time you left - a logical extrapolation of what the future of that moment holds. Of course, the existence of free will allows for the possibility of infinite futures, which is what Doc says at the end of Part III: "Your future is whatever you make it." But time travel into the future takes you to the most likely future of the moment you left.