He should watch the film again.
The biggest unsolved mystery is, what happened to “other Marty”? The one that grew up with cool parents, had a Toyota in his garage, and probably knew how to drive? He probably didn’t wreck the Delorean by driving into a barn. So, where (when?) is he?* And can he return? Did Doc have extra plutonium in the car? He knew what was coming.
And if he does return, I assume he’s going to want his life and girlfriend back!
Marty, the one we follow through the film, is a ghost person. He has no connection to the world at the end of the film. He was not born from that particular Lorraine. He never lived in that particular house. There are probably a million things he doesn’t know about the world. Forget the Toyota, can he even be sure his locker is the same?
And as for blue time traveling widgets, in Superman, there are two Supes existing at the same time. The “original” one out stopping the missile and the time traveling one at the same time saving Lois. Eventually, the first is going to show up trying to save Lois, and be totally surprised that…he already did. But, he’s not the same Supes! The time traveling one remembers her dying, the “original” does not. They are different people.
And now what happens? We have two Supes. There’s no time travel path that gets rid of the dupe supe. There’s no loop to close. Now, like the case of Other Marty, we have two Supermen (Supermans?). We do we do with him? Them?
*my fun theory? He is “George”. He accidentally killed George and took his place (in this reality, they look much more alike.) He’s his own father! Talk about zombies…