I doubt that Twilight will leave her friends to live in Canterlot. At this point her friends are as close as family can be, I'm pretty sure she'll stay in Ponyville, which has become her home.
Also you just mentioned that only royalty can live in Canterlot. Well, that's where she lived before Ponyville. Sure, she was Celestia's student, but keep in mind her brother is royalty as well, so... it's not like the princess plot appeared out of nowhere.
Also, I think going with the princess plot was predictable in a way. I did not see it coming, but if they were going to choose something like a princess plot, I believe this was the way they'd go. I mean, the show is aimed at little girls, and little girls LOVE princesses. So I can totally see what the show creators did here.
Honestly, also see this as somewhat realistic. Things change. Sometimes the change is big, and sometimes it is small. What has happened to Twilight is big, and it will take some adjusting. But either way, it's a change and she has to react and live with whatever life brings her. It's not like she was Pikachu in Pokemon and had a choice. She just changed.
Also it was not just that one accomplishment that turned her into an alicorn. It was all the things she has learned and accomplished over the past 3 years. It was building up to something.
Say if you're leveling up in a game, you need a certain amount of experience, don't you? Well, sometimes you just need as little as 5 experience points to level up. Perhaps this was just the last push she needed before she became an alicorn.