Considering companies like Turbine {Asheron's Call, Lord of the Rings Online, Dungeons and Dragons Online} don't even HAVE 600 employees and still manage to push out content on a steady basis for their games, I wouldn't worry. Even if WoW were to suddenly tank, there's still other games to play that tend to need more of a system to play.

I never buy upgrades based on what game I'm playing right now. I buy them based on the idea that I will be multiboxing something, and if I'm not, I will still have an awesome system for paying anything that comes along for a good long while to come.