I kinda got the idea of this topic when Tera started going F2P. People also have a tendency to leave if a new mmo of that genre pops up. The game that is currently my favorite MMO is Dragon Nest. And when a big game like Tera goes F2P, a lot of users threaten to leave the game they are playing now for the new one.
IMO, I personally wouldn't leave the game. But at the same time, mmo's take a very substantial amount of time. So it is hard to dedicate yourself to more than one mmo. Especially when you are wondering where your loyalties lie when you want to make a microtransaction for an mmo. It does get expensive if you are dedicated to more than one mmo. So sometimes you eventually leave one for another.
Has this happened to you? And if so, what game or mmo was it that you left your old one for? Do you still go back to your old one, or will you go back?