Now, I'm all for various time travel theories, in fact BttF 2 does a great job explaining multiple timelines, but here's the thing about the new life Marty finds himself in at the end of the first movie: His brother and sister are successful adults, led by the good example of George McFly's now-confident and successful living. But Marty is the same. Still a "slacker," at the very least no different despite his sibling's changes. In fact, one could argue he's an abnormality of this new timeline and there should be a successful, different Marty that's a part of this family that is, for some reason, not at breakfast at the moment (Jennifer sure greeted Marty differently when she showed up).
Don't get me wrong: I LOVE this movie! I just like to nitpick at time travel because it's fun. (It's even more fun when my dad gets frustrated about Doctor Who and I have to explain it all to him. Wibbly wobbly and all that.)