well, if MS no longer make operating systems, who'd fill the gap?
seriously, who? i know there are "alternatives" to ms windows (i haven't had ms windows on my pc for so long i can't remember, although i am no linux/mac purist) but are any of them sufficient to replace ms windows in its strongholds? in fact it would be very good if there were alternative OSs ready to step up since then companies etc etc might consider those alternatives even when ms windows is still being sold, accelerating its eventual and inevitable demise.
The only thing companies want out of an OS is a large usable application pool, the ability to Ghost (without any hostname collisions etc.), and some support.
Ubuntu seems to have all these three. Don't know about Mac OS X's ability to ghost it on computers without hostname collisions.
However, if Microsoft would stop right now, it would take at least 10 years before companies switch.
This is for the simple fact of specialised applications running only on Windows, most sysadmins who are too affraid to switch to anything they aren't used to, and a community who would still improve the "abandonware" Windows.