Exactly, look at what would have happened to the movie it they didn't use an alternate timeline and were restricted to canon:
Originally Posted by RodeToad
1. No one can see Nero and live. Since the Federation does not know what Romulans look like until mid way through the Original Series, despite having been at war with them.
2. No new ship. Captain April got first dibs on the ship, not Pike.
3. Different end for Pike. In the show, Pike was debiliated in a much different, and less spectacular, fashion and reduced to a scared freak driving around in a mini-zamboni who could only communicate with a blinking light.
4. The crew could not meet. ThaaAAaat's right. The original crew wasn't all together until several episodes into the Original Series. In the time frame of the movie, they should be a mixtures of cadets and officers who will all serve on seperate ships for many years until finally meeting aboard the Enterprise. Wouldn't be such a cool movie with Lt. James Kirk aboard the USS Republic with no one anyone's ever heard of.
5. Vulcan being destroyed would not be allowed to stand. There would be the stanrdard 'rest button' pushed, which would completely take away the tragedy of the situation. It was even a time travel movie, I was shocked that nothing was reversed.
6. Different character interactions. Kirk and Spock not liking each other at fist. Uhura having the hots for Spock. Scotty transporting away Archer's beagle and being dumped in stupid assignment for it. Chekov as a wunderkid. All interesting character beats that are not backed up with what was previously established. Indeed, watch the OTHER movies, Chekov is a bumbling moron.
7. The Enterprise was constructed in space. This even makes more sense. But it would spoil the reveal of Kirk seeing being construction while on his motorcycle and being tempted. It also gorunds the ship in some more humble beginnings.
8. Nero's ship is completely unlike any Romulan ship..ever. But, it looks damn cool.
If all of those points were followed, and had the movie obey canon (which would also require everything to be reset to the way they were at the end) you have an inferior movie. At best something mediocre like STIII, and at worst, a disaster like 'Star Trek V: The Final Frontier'.