Yeah, I'm sure Epic regrets signing on with the biggest software mover (attach rate) this gen and a measly 11.5 million sales total on the 360 alone. Its also on PC but hasn't sold a whole lot and I don't think it would have fared much better on the PS3.
For me, the Gears games are titles that make you want to buy the 360, along with others of course, and it would have lost much of it's appeal if it went PS360, there would also be slight downgrades in visuals to accommodate for multiplatform (or multi console) development.