u/ElectricMotorsAreBad

So, we see at the start of part two that Marty, Doc and unconscious Jennifer leave to go to the future, when they get there Marty and Jennifer see their older selves, but due to having "departed" 30 years prior, shouldn't they be missing from that future? We see with Einstein at the start of the first movie that for the minute he went into the future he was just nowhere, so why doesn't that cont for them?

I know it's a movie and some degree of suspension of disbelief is needed, but I would like to know if there is an answer.