He's right; people knew World of Warcraft would be a comparatively big deal, but how could they have predicted 9.3 million players? Throughout the history of persistent world online games, we've been surprised again and again at just how large the market for them can be. There are studios out there who think they can take it even further -- BioWare, for instance. Or maybe the industry will fragment into niche games now. The point is, you can't always predict this stuff. That's what makes it so exciting.
In case you were wondering what Roper is up to now, his company Flagship Studios recently released Hellgate: London -- more of a niche title than a mainstream record-buster. Flagship is also working on Mythos, a Diablo-esque-dungeon-crawler.