Except you've thrown away everything that's happened since 1973, some of which you may still want. There's going to be a lot of awful lot of manual conflict resolution to handle - or is there a "world" script that can be run off the new HEAD that plays out the series of events that result from the sentinels being prevented?
I think the idea is that you don't have access to the other timeline, because none of the characters should. All that's left of the dark timeline is in Wolverines's head, which is accounted for!
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.