In lieu of a source, how's about you try to disprove it using data that shows that people prefer to buy Alienware (or other branded) gaming rigs over building their own PC's?
Why are you even presuming a "gaming rig?" I have a large Steam library... on Windows, in Boot Camp, on my MacBook Pro. Personally, I just don't see the appeal of max-settings graphics, so I'm not really compelled to "upgrade" to anything more powerful, no matter how many games I play. (Then again, I also don't see the big deal about movies over-and-above theatre/opera.)