Given the rate of change in OS and GUI environments, support libraries, and other APIs, most desktop apps don't work very well for long these days either.

Of course unlike SaS you can always continue to use an old version indefinitely in a VM if it really matters.

But I find the longevity of the vendor is still critical for supporting new environments and API requirements, and in practice I don't end up discounting the going-out-of-business risk much for desktop apps vs. SaS apps.