Which is probably true-- it's easy to see a future where a game like Dungeon Runners becomes a big hit, and 90% of the people play the game for free (or close to it), and the other 10% of the audience pays for the game by using only the highest level items and gear, and shelling out money for both. But personally, I'd rather go for Raph's other idea-- that smart designers will find ways around integrating RMT solutions by coming up with ways to make RMT unnecessary. I've written and talked about this before-- when it's impossible and/or inconvenient to obtain ingame items with real money, players just won't do it. And no players means no market.
And let's not forget, either, that these are just games we're talking about-- RMT can buy you all the items you want, but it can't buy you great gameplay, and that's the reason we're all here in the first place. If designers emphasize gameplay over simple epic item collection in the first place, there's no reason for RMT at all. Companies like Live Gamer smell money in the air around virtual items, but hopefully (and this is what Raph doubted in his first post) there is still more money to be made with a successful widespread game than just selling the items inside of it.