I personally don't think that PP will overrun GW, but only for the sheer fact that they are completely different in mechanics of the games. GW will excel, I think, because of their fluff and such. They have decades of story build up, and PP will always be behind on that. But, the minis and rules set for PP, I feel, are better, and that means they aren't going to go away. Actually, I think their popularity will be equal eventually, as I think most people will play both gaming systems.
Eventually, what I see happening, is that GW is going to have to change some of their bad habits to stay competitive, just like Microsoft has. Microsoft was the big bad boy for so long, then all of a sudden, Firefox came out as a browser, and Mac started making inroads on the OS front, and Microsoft started bleeding market share. Once they realized that, they actually started improving their products and support, and they are a better company for it.... Heck, I remember when they refused to do updates to IE6, saying it was "perfect the way it was". Man, Firefox really has given them a run for their money since then. That's what I think will happen with GW. PP won't go away, they have a following now, but it will definitely force GW to improve to stay relevant.
A tiny portion of humanity turned its back on mankind in the waning days of the last Golden Age. Over one and a half centuries later, the PHR has emerged from the shadows as an unrecognisable civilisation, its people irrevocably changed. They are no longer simple human beings, they are post-humans - cyborgs.