Real world example for those who think that Windows 10 having a start bar makes it just soooo keen: My father is an architect. He has a $13,000 plotter and scanner in his office that he uses daily. The company that makes the plotter/scanner flat out doesn't exist anymore, it shut down in 2012. The plotter software won't run on Windows 8, Windows 8.1, or Windows 10. Even if you can get the plotter software running, you need a Controller board installed to interface the plotter with AutoCAD, and there are no windows 8, windows 8.1, or windows 10 drivers for that card. Also, the version of AutoCAD he has flat out won't run on windows 10.
Thus, if he "upgrades" to windows 10, this $13,000 plotter becomes a useless hunk of plastic, and he'd have to upgrade autocad for a few thousand dollars as well.
I can list other examples of Windows 10 not being the bestestest OS ever, like this one time I wrote some software for Schneider Electric that absolutely had to run on windows 7 because the version of McAfee their VPN used for security wouldn't run on windows 10, meaning if their computers were running windows 10 then they couldn't connect to the VPN unless their entire backend was reworked. Stuff like that. But no no, keep asking why anybody wouldn't possibly want to upgrade to windows 10. After all, steam runs just fine!