Well, if MS went bust, it would take a hell of a long time before that was a problem with Excel. The huge installed base means that there would be a LOT of people interested in keeping it going, and you'd probably find someone would pick up the pieces and take over the licensing...
Still, interesting to think what might replace it. I don't think the product exists that could completely replace it, although there's plenty for the low-end users (OpenOffice, Google). Would Quattro Pro and Lotus get revved up again? If so, how long before power users switched over?
Denis