I quit CS for Battlefield 2, I quit Starcraft 1 for Warcraft 3, I quit Tiberian Sun for Red Alert 2 and I quit Carmageddon for Flat Out 2. And if I'll have enough fun in either GW2, TOR or Warhammer: Dark Millennium, I'll shut down my WoW account and go play those too.
If any of those MMOs will accelerate or not the decline of WoW, all I have to say is: I don't give a shit, and neither should you.