They'd have to get steam to tell them each time a game was downloaded to a different device so they could invoice. And apple.. and google... and random websites..
Or they make the client phone home each time it's run, which is going to cause its own mess of issues (firewalls, that kind of thing.. some of the corporate firewalls we run our app behind would raise lots of alarm bells at something like that).
Either microsoft buys them, or they simply stop putting unity games on gamepass.
Can't see them paying it (or on what basis, since MS don't have a contract with unity in the first place).