The process of getting to XP lasted from the decision that Win95 and NT4 would share the same look and feel as the start of the process to merge the diverging Windows and Windows NT approaches into a single OS with verying levels of facilities - home, pro, etc. XP was the end result.
Vista, from my experience of it, was pretty good. Ran faster than XP on the same hardware and handled audio work better. It suffered because it looked different and because beta versions prompted all kinds of silliness by people posting stuff about it on the internet, much of which wasn't even true. Rumour+gossip+journalists often = accepted fact no matter what, so bye-bye Vista, hello Win7 (using pretty much the same code as Vista with a more old-fashioned interface).
To me the really big MS switches at a low level were shifting from 16 to 32 bit then making 64 bit OS versions available and the shift to "NT for all" because their previous "domestic" versions of Windows, like Windows3.x, had no concept of user-level security at all. At each stage some applications no longer worked (or the installer didn't), but most did.
For MS to move to a completely different system entirely, be that Unix-like or something else, would, if possible, take many years and I suspect there'd be few takers for the early versions because there'd be little software for them. The biggest customers would probably be the last to make the switch for the reasons I've given, and it's the big customers who hold the greatest clout - e.g. MS agreeing to continue to maintain XP after the announced "that's it" date. The need to learn the new MS OS then reconfigure their LANs and WANs accordingly would be a huge off-putter.
Apple's business model is very different to Microsoft's, largely relying on hardware sales and not just in the laptop/desktop market but they have a strong presence in the tablet, media player and phone markets as well. Apple give away OS X, their equivalents of Word, Excel and Powerpoint and provide a decent package of applications with the OS which integrate very well with those provided with iOS. The server version of OS X costs the price of a CD. Logic Pro X is very cheap for a "real" DAW etc.
Apple can use the software as a way to encourage purchase of the hardware. MS, besides a few peripherals and their attempt to get into the tablet market, rely entirely on selling/renting software. Big difference.
I'm not saying MS won't decide to go down a completely new road, they might. I just find it hard to imagine them doing so, at least as anything other than a very, very long term process.