Unfortunatelly, the "staying power" not just depends on the software and license. Both may be a prerequisite, but without constant strong interrest/support from the maintainers or the audience, those nice downloads will simply vanish from the surface of the planet. The internet is not a reliable longterm datastore. Here's something to try: go to the websites of your favorite opensource applications, note how "big" the interest from the maintainer/fans is - then check for binary and source downloads of older versions
And no: The newest version doesn't usually contain all the benefits of older versions. An app that may once have been compilable on X, may in its current version no longer support that target.
Bottom line: unless there are enough people interested in something or you do have an own backup, software simply stops being available.
Similiar conditions apply to future development and porting - nothing happens unless someone is interested enough to do it. Just because a lot of people COULD do something doesnt automatically make that happen. This argument is what mainly backs my earlier comment about what kind of project-style i prefer: Without stable interest, not much gets done.
"Software" doesn't have staying power. People's interest in a particular software is staying power - the tech just sets a cap on what can be done IF the required interest and manpower is there.
As for microsofts interest in .NET: NET and its related frameworks are not just important to MS - they are perhaps more important to them than anything else. Even more important than windows. Middleware and frameworks are microsofts desired monopoly, and NET is central to their current plans. They gave up on the OS-monopoly many years ago: Sure, windows is still important to them, but mostly as a leverage to bundle middleware. To MS, it's not about "does it run on windows?" but instead "does it run on the middleware that is controlled by us".
I'm kinda amused that so many people haven't noticed this happening in all those years. Why on earth do you think that MS is pushing .NET so hard and wiring it to other MS middleware so that you get locked into a dependency-web of MS-middleware? I mean, its not like MS made any secret about this intention, and high profile people in the industry predicted this years ago (anyone remember that Joel on Software article "How MS lost the API-War"?).
CIB's remark regarding support and promotion was more spot-on than he perhaps realized. This isn't about OS-wars - it's about the "Sub-OSes" that run on top of existing OSes and abstract those away. It's about "software-platforms", not operating systems - always has been (back then, it was just that the platform (the windows API) was close to the OS), and still is.