My 2009 Dell Latitude is currently running Windows 10. So sure Windows Vista is no longer supported but I upgraded for free along the way as Microsoft offered the updates. So I wouldn't say that Microsoft no longer supports old devices because they don't support the old OS. Unlike Apple you can upgrade to the new OS when the old one is no longer supported.
Well yes, but perhaps not everything on the computer will continue to have device driver support with the new OS. I couldn't get a working device driver for the WiFi on my Dell and the touchpad became flakey, so I installed Linux. I did the same when Apple stopped providing security updates for my (then 9 year old) Macbook.
For both PCs and Macs, I'd start thinking about Ubuntu after 7 years or so.