It is very interesting what would happen if you left the present and reached the future. In The Time Machine the time travelers asks about himself and people claim he went missing when he left on his journey through time.
It seems like an BTTF2 Marty was taken away in 1985 so his kids and everything couldn’t exist in 2015…
But the existance of his kids might just be evidence that they will later return to 1985 to do all those things! This would imply that everything they are going to do have already been decided!
It became clear the the future is NOT set as Doc claimed the future hasn’t been written yet. So how could they travel into what does not exist? Even when Marty went to 1955 he must have been from the future from some people’s point of view?
The future they travelled to might just have been one possible future! In the quantum many worlds theory there are an infinite amount of futures where every possibility occurs…