I agree that the casuals eventually might turn into hardcore players and help establish new genres. The thing is, Doom wasn't. Because people waited after Wolfenstein 3D on Doom and discussed it online in usenet (early predecessor of internet forum) and the demos they had seen so far. That is core player habit. The anticipation for Doom was that big, that it spawned the "Smashing Pumpkins into Small Piles of putrid debris" meme, which eventually was acknowledged by the finally releasing Doom as a cheat code.
It's not surprising that Wolfenstein 3D fans wanted to play Doom. However, Doom was a much more popular game. It definitely sold a lot more. Also, I had never heard of Wolfenstein 3D until years after it's release. But I couldn't escape Doom when it was a new game. So many people were talking about it, and it was not the people I normally considered gamers. Not only did it sell a lot, but many times more people were playing the free shareware. Doom had reached a casual fanbase that Wolfenstein had not. The PC was not really known for having good action games before the FPS genre emerged. Doom was played by people who had a PC but weren't into either the non-action PC games at the time, and they weren't into console or arcade games either. It had reached a new set of gamers.
This is not too different from WoW. I am sure there were some hardcore MMO players that wanted to play WoW when it released. But WoW reached a subscriber base far beyond what previous MMOs had. Most of these were casual players who had never tried an MMO before.