The short answer to the title question is that it'd probably make for a pretty terrible movie. But the people over at College Humor decided to show us the answer instead of tell us with an animated short. And their timing couldn't have been any better since the actual Back to the Future Day is slowly approaching.
The animated short (above) is funny in that dark, sad, "oh man everything sucks" kind of way. And that being said, I think that's why I like it so much. We have Cartoon Marty McFly making references to Back to the Future 2 and every time he asks another question, Doc Brown just shoots him down to tell him the reality of the situation.
Actually, now that I think about it, it's kind of bleak. It's like a sad realization that in 30 years (if we go by the film's narrative) we haven't made much progress as a society.
Let me preface with this though: I don't fully subscribe to the idea that the advancement of technology is an overtly negative thing (if I did, I definitely wouldn't be on the Internet right now). And that's the picture that the College Humor short paints.
We see a group of people on those handlebar-less segways taking Vines of themselves while McFly hates them for it. We have all these pop-culture references that give us a holier-than-thou sort of feeling to the short. But I think the bleakest thing about this is the way Doc Brown reacts to Marty when he's trying to use garbage as the fuel for the time-traveling DeLorean.
When I first watched this movie (I was very young), I always thought that was the coolest part of the movie. It was silly, it was ridiculous, but it was also super cool. I remember having tiny baby hamster dreams about picking garbage up and sticking it into my Dad's Pontiac and we'd drive all across the country. But instead, gas prices are ridiculously high sometimes and that alone influences my decisions when it comes to moving anywhere.
It's supposed to be "the future". Instead of being this science-fiction wonderland, everything feels the same except things are more expensive, people are less genuine, and everything is a lot smellier.