just got my copy of windows 7 so it can use updates again so i am keeping windows 7 until the end in 2020. i also have a copy of windows xp which i still use sometimes for software which doesn't work with windows 7.
For the Win 7, I won't argue, but I can't see why you don't do a clean install of Windows 10, even by temporarily swapping out the Win 7 disk for a bare-metal for the Win10 install. Use the Win7 product-key to activate the Win10, and swap the win 7 drive back in. On mine, I just made space for a separate boot volume of Win10 so I can dual boot to either.
You could even do the install that way, saving the installation media on USB, and then wipe the HDD/SSD after activation. From that point on, you wouldn't have a problem ever doing a clean install of the Win 10 from the USB and re-entering the Win 7 product key. But you want to activate it now -- before the deadline of "free Win 10."
That way, you won't have to purchase it for that particular machine when you decide to change up. of course, the machine could be so old, you might not even anticipate ever using Win10 on it, but I wouldn't pass up a freebie, nevertheless.