I don't know what I was smoking yesterday...
...but I decided to try to upgrade an old Windows 7 HP Pavillion laptop, on the basis I'd just put an SSD in it, and still had the original disc in case it went tits up.
This was despite trying out Windows 10 on an even older HP netbook about 6 months ago. I kept finding it switched on after doing upgrades in the middle of the night until finally one upgrade got stuck, and after rebooting, it supposedly rolled itself back, then it booted to an empty desktop. Attempts to repair it resulted to it not booting at all, so now it runs Mint+Mate faster and better than it ever did with any flavour of Windows.
On the laptop just like with the netbook the automatic upgrade downloaded 3GB, took hours to get to 87% then stayed there indefinitely, and on rebooting returned to Windows 7. I still had the media creation tool on an SD card from trying to fix the netbook, so tried that, but luckily it wouldn't accept the 100% genuine licence key from the sticker on the machine, and that brought me to my senses.
Now W10 is no longer free, I'll never be tempted to try it again.