Good luck getting any grandma's to know how to even turn on a computer let alone formatting it lol Those jobs are sent to people like US to fix and there for "should" know how to fix them.
You can pull lots of different % of the internet, can range from 22/33/40/48% depending on where you look and no its not faster adopted then Windows 7 (yes for the others couse they sucked!) as Windows 7 only took 2 yrs to be on top, W10 is still behind after 3yrs and ONLY going up because we are getting it forced down our throats.
Fact of the matter is Windows 7 has only lost a huge (well not really) support from MS is because there trying as hard as they can to move as ALL to W10. The fact they stop updates from working on not just 7 but 8.1 is pure BS and they should be taken to court over, 8.1 still has FULL support but MS says to you and everyone else FU!. DX12? meh who gives a fark, small amount of games that even use it, and Vulcan kicks its ass anyway. Wait what? you think XP/7/10 sucks? wtf? are you high? there like the best OS's that MS has even come up with (W10 still needs work), there not used the most out of all OS's just for the hell of it lol. You think Vista, 8/8.1 and ME was a better OS? dear god you really need to get out more and look at the rest of the world if you think that lol.
The only reason you have seen a sharp drop off across shops of 7 is because MS pulled all the 7 OS's off the shelf! Sent back to MS and destroyed, and thats directly from Harvey Norman them selves. Is it because everyone likes and wants 10? fark no! its because MS has made it near impossible to get an older OS now. If you could still walk into a electronics shop here and buy ether 7/8.1 or 10, 7 would outsell the rest by far.