I think you're missing the point here. I don't think matters was trying to make an argument that everyone should play both at the same time, but that more than one MMO can exist at the same time and be a financial success. Let's say for the sake of argument that there were 10 million people total playing MMOs, one MMO has 5 million and the second MMO has another 5 million, this would make both MMOs successful.
You and many other people might not play 2 MMOs, but that doesn't matter. Not every single game is going to appeal to all the same people, meaning one MMO can have X amount of players and be successful while another MMO might have Y amount of people while still being a financial success. It is perfectly possible for SWTOR, WoW, GW2, etc to have a following that brings in the moolah for their respective developers making them all a financial success, and in the end that's all that matters to these companies is the money they're making.