The privacy issues are certainly a concern and showing direction that Windows is evolving into. Probably an SaaS (Software as a Service) ie. Cloud, operating system. This will make it so that your data will need to at least in part be stored by MS. It doesn't mean that they have to give themselves permission to look but I'm sure there's no way they won't at least try to build lots of analytics based on it, meaning they will have to look, even programmatically. What better way to test the waters than this? Just look at Cortana and what that means to privacy. If you want all of your data that interacts with Cortana private, you have to disable it.
On to the other stuff. I haven't upgraded. My wife tried and it got stuck at 30% for hours and she restarted. Windows blue screened and I had to rebuild the BCD. Not by the awesome bootrec /RebuildBcd. Oh no. Windows boot loader couldn't find the sector of the harddrive c: was, much less c:\Windows to boot it. I had to do a checkdisk to rebuild journal entries, then make the partition active with Windows on it, then force write the boot sector and again, not with the awesome automatic bootrec /FixBoot. Eventually, after manually rebuilding (not complete super manual - which I've done before by physically writing the boot file, creating a new volume GUID, and then forcing my new parameters into the bootloader) I got Windows 7 to boot again.
I'm sure it would be fine on my own computer that I build myself but I still haven't wanted to upgrade. If I wanted the "new" feature of multiple desktops, I'd use Linux (I do!). If I wanted DirectX 12, I wouldn't care until I had a DX 12 video card, if I wanted faster boot, I'd force logoff of the user and put it in hibernate anyway. There just isn't a whole lot of reason to update with one exception. Some Visual Studio functionality is locked into Windows 10. But honestly, if my programming environment is important to me and it is locked into an OS, then it isn't a very good development language to begin with.
Seriously though. When the biggest features are the: Start Menu (I use Windows 7 for Windows anyway), touch centric UI (I don't have a touch screen and I use a desktop), Cortana (I didn't use my own voice command assistant I built for Windows 98 in the 90s, so I doubt I'd give up my private data to do so now), universal apps (uh yeah, I can use executables, yay!) and Secure Boot which does the exact same thing BIOS since the 90s have been doing but this time in a locked down environment where MS literally holds the (private) keys to allow only certain operating systems to boot (UEFI) then I can't really see a need to upgrade.
Sure there are also the aforementioned virtual desktops and supposedly an apt like software update system, both taken from circa 2002 Linux but if I wanted that, I have that.
The other issue is that the first upgrades are going to be the worst. MS will need to push patches to fix problems. Better to let the early adopters face these problems and let you upgrade hassle-free.