Supported lifetime? Experiment in progress ....
I'm going to scrape together some spare components and build myself a test PC. I will load a legitimate copy of Win 7, and let it "auto update" to Win 10. By all accounts, this ought to get me on to the "free for life Win 10 as a service" package. I'm then going to start messing about by adding a second HD, changing memory, graphics cards, software etc.. and see just how long it lasts before it borks.
My guess is not very long at all, and that the "supported device" that MS have in mind is the one that came with Win 10 pre-installed - perhaps ideally a Surface where you can't change anything anyway, or even repair it when it breaks. My fear is that Win 10 will recognise the device on which it is first installed, but not the "new" device created by changing or upgrading hardware.
If I am right, and upgraded devices become unsupported fairly quickly, then users will have to go out and buy a new one, with a distinctly un-free copy of Win 10. Shock - horror - surprise!
Let's wait and see. In the meantime, my working environment will continue to be Linux, and I really cannot see anything coming out of MS that would ever make me want to go back.