As many have mentioned, Windows 8 failure was purely from a UI standpoint. Any Windows users who have used it with Start8 or Classic Start can attest that it's faster, more stable and overall better than Windows 7. This is also the first Windows release under Satya Nadella (Ballmer Free!) as well as with a new lead for the Windows faction of the company (I have read many an issue with Sinofsky being a terrible lead for Windows 8) so I think 10 will likely be the "best" Windows we have seen yet.
Now to speculate, my belief is MS will continue its cheap/free licensing of Windows 10 for tablets and phones. They will also offer a free/cheap upgrade for Windows 8 users to upgrade to Windows 10, and unlike Android tablets MS can push that right to users without having to go through the OEMs (not sure about Windows Phone 8) so we'll quickly see Windows 8 market-share plummet and 7 and 10 will be the majority of users.
Now despite all this Windows will likely still drop marketshare on the desktop and will gain a bit on the mobile side. Linux folks will still use Linux. Apple folks will not be dropping their Macs and iPhones to get Windows 10, but that doesnt really matter. If Windows 10 is technically as good/better than 8 and get' the interface right (which it seems like they are doing enough to satisfy desktop users) then they will keep their Windows userbase happy and likely Win10 will be the one we see business move off Win7 and right now that's likely job #1 for them.