I doubt the climate is right for MS to throw money around in such a manner. They seem happier to focus on DLC more than full titles currently, or timed exclusives.
While sure fire earnings are attractive, developers will also have to consider their long term market position as well.
They need fans, they need a lot of people playing their games. Going multiplatform, making the same money and having more players is preferable than making the same money off less players as market perception and brand takes a knock.
MS have the money of course, but I think currently PS3 sales are too strong to ignore and I'd expect most developers to be wary of taking such a route. As more and more developers are saying - right now multiplatform is preferable for expensive HD games.
Try to be reasonable... its easier than you think...
