Honestly, if that happened that would essentially be Microsoft conceding defeat in the console market and you may very well see the death of the Xbox brand shortly after I feel. If the priamry reason to buy the machine can be had elsewhere then what's the point of having the machine anyway?
Doubt they would actually do that now though, I know that first two were released on the PC but that was back when their Xbox was getting routinely trounced and they just wanted to expose as many people as possible to Halo, since 3 they have been console exclusive and wisely so if Microsoft wants to continue shipping the Xbox brand, that said even though they have been decently reviewed and the sales have been decent the last few entries in the Halo franchise under 343 haven't exactly lit up the Xbox brand as the series seemed to under Bungie.
Playstation gaming for 18 years this 2012 
