^^^ Perhaps until I wanted to use my elderly (but excellent) film scanner, Togo, that's not got a Windows 10 driver available.
Or when I wanted to run my XP-based programs, such as Photoshop 6, QuarkXpress4 and Microsoft Publisher 2002, which will all run well in Compatibility Mode on Windows 7 (but might not run under Windows 10)? If they don't work it will cost me £799 to buy the latest version of QuarkXpress, £120 to get Microsoft Publisher and £599 for a version of Photoshop that's as fully-featured as Photoshop 6.
So my 'free' upgrade could cost me over £1500 in new software, plus a new film scanner (as well as additional peripherals, such as a video-grabber, which also aren't supported in W10).