I doubt anything is going to happen at all. TESO has been known for a long long time; people who were interested in playing it were going to play it regardless. Those people who still want to check out ARR will do the same. The timing might make a few players have issue if they're only able to play one game, but in the end quality, not timing, will decide which is "the victor" if you want to break it down to that.

Nothing will happen. Every time a new MMO is announced a thread like this pops up.