I'd go with that. The PC is already pretty inundated with shooters, some of them very good and with well established multiplayer scenes. It'd be a tough sell, especially for Microsoft. All the Xbox hype they built up has only made PC gamers bitter and less likely to buy.
And to be honest, I don't think Halo has any trump card to play. It's got a few interesting design decisions - all ripped straight from Starsiege: Tribes of course - but regenerating shields aren't going to set the world alight. Meanwhile, it's squeezed between UT2004, Team Fortress 2, Counterstrike, Call of Duty 4, and the infinite creativity of their mod communies.