To me, the beauty of the story is that, while we as viewers do not know what has transpired in the universe between 1973 and present, neither does Wolverine. Further, since he stills ends up at the School with his friends alive in the present day, we know that a lot of character development has necessarily happened, because we saw analogous stories in the earlier movies from the now lost timeline. So while the events of the previous movies are now no longer canonical history, they are as real to Logan as they were to us, and their emotional impact still affected Logan's life in a very real way. Meanwhile, going forward the script writers are no longer slaves to continuity, which, after the third film blew up everything, greatly restricted what they could do with the main cast of characters in the present day setting.
In my opinion, so many "time reset" plots (think Star Trek Voyager's Year of Hell) leave the viewer empty because they truly have no lasting consequence on the character's we love. SF Debris (http://blip.tv/sf-debris-opinionated-reviews/voy-year-of-hel...) made the incredible suggestion that, in that Star Trek story, imagine what if, due to some kind of quantum chronometric mumbo jumbo, one of the crew members (say, Harry Kim!) had survived the big reset with their memories and experiences of the year intact. But instead, apart from some shallow insights into the characters in a "what if" scenario, it was as though it never happened.
You see that in the film, where he meets some characters he previously knew but didn't expect to see. But we don't see the other side of this: there are surely characters in the new timeline that Wolverine is very close to that he will have no memory of. I guess that sucks for those characters.
Wolverine's return to the new future essentially kills off the one that had been living in that timeline since 1973 and nobody cares.
The Professor was in Wolverine's head back in the day and might have learned a few things that needed to to be done.