I wonder which version of the original timeline the OP means. There's the one where the Enterprise is a starship under the authority of UESPA, and there's the version in which Enterprise is a Starfleet starship. There's a version in which there were only 12 Constitution class starships, and there are other versions that indicate that there were a lot more than that. The comics have depicted a version of the original timeline in which Captain Kirk and company fight off their mirror universe counterparts before taking command of Excelsior at some point between The Search For Spock and The Voyage Home. There a version of the original timeline in the novels which hint that there was a second 5 year mission between the end of the first 5 year mission and the first Star Trek movie.
Then again, someone has pointed out that these are all just fiction. It seems to me that the happiest group of fans are the ones who post on Doctor Who forums, because they know there is no point in getting fussed about the consistency of the show's continuity because it may be changing everytime the Doctor lands somewhere and gets involved with the locals. The Star Wars fans were kind of happy for a while, until the new Clone Wars series started messing with what had already been "established" in previous books and comics. I've seen Star Wars and Star Trek fans proclaim their pride in their franchise of choice on the grounds that it is internally consistent. This is a pathway towards unhappiness.
An author a few years ago opened a Doctor Who novel thusly: "Every word of fiction is true." Arguably, that's not necessarily true, but it could be. I look at it this way, any work of fiction can be true for the person enjoying it, in the moment that they are experiencing it. I've seen the new Star Trek movie, and in the days and months following that, I've watched episodes of the old series, and enjoyed them on their individual merits. I've enjoyed reading through the old Marvel Star Wars comics, and believe me there is a very strange version of Jabba the Hut in those stories. And because I hate all of the stupid retconning to get around perceived awkwardness, I read them, and take them at face value in the moment. Within the confines of that comic series, Jabba is a humanoid walrus. Believe me.
Does anyone complain about how there are vastly different versions of continuity for G.I. Joe in it's original marvel comic run, compared to the animated series, or the new movie (which I found to be a drastic departure, but no one else cares, do they, so why should I?) What about Transformers? There's some pretty big differences going on there, between comics, animated series, and the new live action movies. How many times has the origin of Superman get retold in just the comics alone? There are several additional versions of his origin between all the TV shows, and the movie series. These franchises are reinvigorated by being reimagined, retooled, rebooted, and told anew.
Some people have complained that the writers of the new Star Trek shouldn't have even bothered with showing how the old universe still exists in relationship to a potential new version of history. I actually think this would have been a worthwhile way of doing it, but I confess I do like the time travel antics, and Nero was interesting enough as a villain, but just as interesting as an adversarial temporal incursion. The only thing that matters is giving writers room to take the original Star Trek crew in new and potentially more meaningful directions. I'm not saying the old crew weren't taken in meaningful directions, they were. But now there is the potential to take things further along than they ever could be.
The point is about maximizing the potential of the characters and situations. A writer just can't be expected to memorize all the stuff that is contained in the Star Trek Encyclopedia, just so they can get all the details right! It's great stuff if it inspires a writer to think up a new story idea, but it's junk if it hinders the storytelling process because the details aren't exactly right.
Regardless of whether we follow on the logic the new movie graciously offers us for believing the old universe still exists, think about it: when your watching an old episode, at that very moment, doesn't it exist for you, in a sense? We know that it's fiction, but when we're in the moment it kind of exists, doesn't it? And then you can go back to the new movie, loath the possibility that it just overwrote the old universe, then go back and watch an old episode and once more, be in the moment.
This is suppose to be fun! When your watching or reading something, let yourself believe in it! When your not watching it, ponder on discontinuities, and, if it's fun, try and come up with your own explanation for inconsistencies. And if there's no reconciling it, set it aside and remind yourself that it's all fiction. And then let yourself be absorbed by the story when you read or watch it the next time.
Why is this so important? There are fans of other fictional universes who are very happy.