The biggest problem with this statement is that he seems to be defining "MMO" as "WoW Clone". Even if games might have some original content, only defining games that adhere to all of the conventions of World of Warcraft as MMOs is a very narrow view. What about League of Legends? What about EVE? (not saying Dota because that arose from a fan modding institution that doesn't really exist anymore) What about Team Fortress? What about Farmville? Hey, what about CALL OF DUTY? (the Modern Warefare template- not the old WWII series). All of these are successful games that are massively multilayer online games. And if you say "well that's not what we mean by MMO" then that just shows that you are overly shackled into thinking of games as needing to tick off all the checkboxes of most popular game of the moment.
So when you say "MMOs (read WoW clones) aren't successful in the US" maybe you should consider that's because players interested in playing a game modeled off of World of Warcraft and adhering to all of it's conventions quirks are already playing, have been paying for, have invested time in and have friend on WORLD OF WARCRAFT. They're not going to switch and dump all that because you made a passable extremely similar game. You need to make a game that offers something different-that's what all the successful games I mentioned above have done.