Yeah, you can call that stupid, but I am not stupid. But regardless of your insult, let me talk to you.

I am not sure which Microsoft product you are talking about, but certain Microsoft products are indeed charged this way. It is called an OEM license and while typically every OEM license is negotiated on different terms (per license, per revenue, per download, per install, per duration of usage, etc.), the basic idea is usually that some amount of money will be made by Microsoft proportional to how much use your product is getting. In the Enterprise world it is also common to charge by how much value the user is getting out of the product, which whole sales departments are trying to figure out on a case by case basis with complicated excel sheets. I mean, it is not like Unity invented this model. In fact, Microsoft got as big as it got by selling a pay per copy version of MS-DOS to IBM.

Unity is an ingredient that makes games work. The game is made with Unity and is shipped with the Unity engine packaged inside, just like any other ingredient. So explain to my why Unity can not define some metric which will highly correlate with the amount of usage and charge based on that metric?

So what I get from Unity's site is that they will charge per download. So yeah, potentially you can download a game three times to three different devices or even to the same devices you've wiped. But I would claim that generally speaking the number of downloads is a good indicator of how often a game is used. If you don't like a game you are generally going to download it only once. If you really like the game you are likely to download it again and again to new devices and after wipes. It isn't perfect for sure, but every other metric you can come up with also has a fair share of problems. Maybe you tell me which one is the one that does not have any problems and is still simple enough to bill upon?