Detta svarades på officiella BTTF FAQ som gjordes av manusförfattarna Bob Gale och Robert Zemeckis :
1.13: When Doc takes Marty and Jennifer out of 1985 and brings 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 Back to the Future Part II. We really thought about this one for a long time, but we finally decided that after the set-up of Doc saying "Something's got to 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 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 Back to the Future 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 Back to the Future 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.
Jag tror att detta projekt av den "logiska extrapoleringen" av det ögonblick du lämnade kan också förklara varför Einstein inte stöter på en äldre version av sig själv - för att Doc aldrig hade några planer på att skicka tillbaka honom att leva ut den minut han hade missat, medan det alltid var en del av planen som Marty skulle gå till 2015, lösa problemen med sina barn och sedan återvända till 1985 för att leva sitt liv normalt, så den "logiska extrapoleringen" tog det med hänsyn till.
Med tanke på detta svar är det intressant att spekulera om huruvida den 47-årige Marty 2015 skulle ha ihågkommen att ta en version av en resa till 2015 när han var 17. Kanske skulle han komma ihåg en mer logisk / sannolik version där Freakolyckan av Biff överhögt om almanacken hände inte, så hans minne skulle vara att de hjälpte sina barn 2015 och sedan återvände till 1985 och gjorde aldrig mer tid på resan.