Yes, you can use the same Windows 7 key on multiple Windows 10 computers.
Yes, you can use the same Windows 7 key on multiple Windows 10 computers.
There's a subtle but important distinction that must be made here - in the event you use an old Win 7 key to generate multiple licenses for Win 10 and use all of those simultaneously, you're not using one key on multiple computers, you're using a different key on each computer, each of which granted officially, legally, etc. by Microsoft themselves. The original Win 7 key is used on one machine, the upgrade is done which generates a new key for Win 10, leaving the original Win 7 key no longer in use and able to be used on another machine. This can be repeated so long as Microsoft chooses to allow it, and no further. For now, they allow it. At no point is any one key in use on more than one machine at a time. If Microsoft did not wish for this to be possible, they very easily could have prevented it in one or more of a variety of ways. The most obvious of these would be to have the key generation algorithm use nothing but the source key as a seed such that any Win 7 key could only generate one unique Win 10 key, and additional attempts to upgrade would thus result in the same key, therefore disallowing use on multiple systems concurrently.
Thank you for your feedback. I understand you think Microsoft is mainly focused on encouraging stock purchases through Windows 10 licenses. In my view, they generate about 80% of their revenue from other sources like advertising. I’ve heard rumors suggesting they might consider making Windows 10 free for everyone, though they’d likely keep OEMs paying.
Just because something functions doesn't guarantee it's allowed. I can modify a program and claim ownership or simply download it and assert it as mine. My car can reach 250kph, but that doesn't make it legal. Many actions I take in life aren't automatically legal either. The final part of my post warns against sharing software between devices, which is stricter than what OEMs typically allow. Some guidelines suggest only one instance at a time, and others mention bypassing technical limits. What seems fine technically can still be considered illegal depending on the context.