Woah, woah, woah. Let's not get ahead of things here. If Windows 7 isn't a success Microsoft still won't go under. Vista didn't cause it to fail, and while it may not have been as successful as MS wished, it still sold well. Though sales are now slowing, Vista still sold 28 million copies in the quarter to September 2009, by comparison Snow Leopard was expected to sell only five million copies in its launch quarter.
Vista had problems, yes. I wrote, before it launched, that it needed to be streamlined (no one at MS listened to me, which again shows they should). On the other hand, I moved to Vista nearly two years ago and I've found it a good OS, it's robust, reasonable quick and very usable. Provided, that is, you have the hardware to run it. Get a good machine and it works well, use it on something that previously ran XP and it'll be slow as a dog.
Microsoft's problem was being over ambitious, they thought everyone would buy new machines to run Vista, they didn't, they though everyone would buy new peripherals to use with it, or that manufacturers would dedicate lots of time to building Vista drivers for existing hardware, neither of which happened, so everyone got a bad experience. That's what killed Vista, not the OS itself.
As Windows 7 is very similar to Vista, but has seen my suggested performance improvements, those problems won't exist this time around.
As for the 'last OS' I don't think that's right either. The more people use the cloud the more we have seen it's failings, mainly in security, I don't think you'll ever get rid of the OS and local applications.
I've used all three of the major operating systems and I keep coming back to Windows. It's easier to use, it has more applications available, it's generally better supported, in my experience. I recently used Ubuntu and while linux has come on, it's still a very long way from mainstream adoption (installing apps that haven't yet been Ubuntu'd is a complete pain -- "I have to download, then run some commands from a terminal, then install -- what?"). While I like the price (free) the amount of time saved using Windows means the Microsoft tax works out quite well.
The king is far from dead.