I just don't like to think of people giving up on perfectly serviceable hardware. You would be suprised at the age of machine windows 10 will run on. I have it installed on an old packard bell easynote that's running an Athlon II P340 with mobility Radeon HD4250 graphics. the graphics chip isn't even supported by AMD any more but windows 10 found a driver for it and it works fine for browsing and watching youtube vids. I can still run some older games on it like GLquake.You speak perfectly right things, mate. But when Gonegirl's PC doesn't have a BIOS, I'm totally agreed with her: she won't be able to install Windows 10 on it, actually she wouldn't make such a PC bootable at all.
We could make the point on existence of such a thing that called backward compatibility. Or we could point on system requirements of a certain Windows version, certain applications etc. But who knows what can happen in the world of computers that lack "the registry" thing?
Although in that reality where I live, people do install Windows on everything, and the newer OS is, the more are chances to update seamlessly. And vise versa: the older the OS is, the bigger trouble you discover trying install it on your shiny PC (e.g. Win7 on CFL or Ryzen).