Every Visual Studio for Mac license sold means one less Windows license sold. If they make the transition away from Windows easy, less corporations would want to enter corporate licenses. Mac users would consider using iWork instead of Office. That move would, ultimately, erode Microsoft's market dominance, then their share and the slide into one-more-competitor condition would be inevitable.

If they ever do that, it'll be because they think Macs and other OSs don't represent a threat to other Microsoft software.