Quit a PhD for another PhD
Quitting a PhD because you realise afterwards that it's not the right path for you is fine. A fellow student during my PhD did so (due to homesickness, primarily), and immediately started a PhD at a university in his home country. The supervisor/group leader/etc were disappointed but didn't hold it against him.

Quitting because you only accepted the position as an insurance policy is a very different matter. If it's not for personal reasons, such as those above, leaving mid-course for another university will be seen as a very selfish act. Even more so if anybody finds out that you were hoping to do it before you even started. From your description it sounds like your field is relatively small. That means people tend to know each other, which means this could very easily hurt your reputation far more than attending MIT (or whichever other prestigious institute) would help it.

In short: you shouldn't accept a position unless you intend to stick with it. You have an offer on the table, you can either accept it or reject it. If you accept it, the right thing to do is to withdraw any other applications which haven't yet responded. If you aren't willing to commit to it, the right thing to do is to reject the offer. There is a third option, too: you can ask the European university if they'd be willing to grant you an extension to the decision deadline. How likely this request is to be accepted strongly depends on many things, including national and institutional cultures and the length of deferral you're asking for.