First off , lets start off with the plot to begin with , it makes no sense
Doc travels to the future of 2015 , finds out that Martys kids end up setting off a chain of events that ruins the McFly family. He returns to 1985 to tell Marty and Jennifer about it - so why do they need to go to the future to prevent it ? . This is where Part 2 jumps the proverbial shark more than the other 2 movies. Lets jump to Part 3 -the fax that Jennifer is holding at the end of Part 3 which was a fax of 2015 Marty getting fired in Part 2 magically erases- . Doc explains that their future isn't written yet hence the paper erased , of course the paper should have disappeared too in my opinion just like the tombstone disappearing in Part 3,.
So with that scene of the paper being erased, it would mean that 1985 Martys timeline hasnt been written in the future. So in retrospect , why would you need to go into the future when you are in your past and could easily prevent it from happening since you already know its going to happen in the future,
That's the whole problem here , you obviously have alternate timelines . When Marty and Jennifer of 1985 travel to the future of 2015 , how could they exist if they disappeared from 1985 , there would have to be an alternate timeline where they didnt go into the future and stayed in 1985 and got married and such until 2015 when 1985 Marty and Jennifer from their timeline arrive, so it makes no sense for them to go into the future since it wont be their future anyway but an alternate trimelines future.. Totally incoherent plot - the future doesnt exist yet at the end of Part 3 so that writes off part 2 . Sheesh.
So with that scene of the paper being erased, it would mean that 1985 Martys timeline hasnt been written in the future. So in retrospect , why would you need to go into the future when you are in your past and could easily prevent it from happening since you already know its going to happen in the future,
That's the whole problem here , you obviously have alternate timelines . When Marty and Jennifer of 1985 travel to the future of 2015 , how could they exist if they disappeared from 1985 , there would have to be an alternate timeline where they didnt go into the future and stayed in 1985 and got married and such until 2015 when 1985 Marty and Jennifer from their timeline arrive, so it makes no sense for them to go into the future since it wont be their future anyway but an alternate trimelines future.. Totally incoherent plot - the future doesnt exist yet at the end of Part 3 so that writes off part 2 . Sheesh.