Some would argue that Microsoft got to the top of the pile due in no small part to maintaining backward compatibility. By doing so they lower the barriers to upgrading for most of their users, who don't want to reinvest in new versions of applications that can't make the jump.
Vista strikes me as a reasonably good OS crippled by the need to support the entire win32 legacy. It's too bad that Microsoft can't take 2000 of their best programmers and turn them loose on building a new OS that supports .NET and nothing before; as it might be quite worthwhile.
However; Microsoft the company seems to be slipping into the senility phase, thinking that their spot at the top of the pile is a god given right and that they should stay there forever. They seem to have forgotten that it's capitalism, it's dog eat dog; and it's their turn to be a dog's breakfast.