Proportional does not mean one equates the other. It means that while one goes up, the other goes up as well. It’s not going to be some constant factor and it’ll depend on the game, but you should expect that for every license you may have a handful of installs. You simply need to account for that. If you would have to come up with a mathematical function that estimates the number of installs your game is going to have and you know the amount of users, would you use the amount of users as a coefficient in your function? If so, then that means it is proportional. If not, then please enlighten me how you would guess the number of installs without the number of users.
Now the next question is, is it fair? Why not? One business model will be the license model, but another business model could be based on usage. Perhaps long time users are buying in-game items, doing upgrades, looking at ads, are willing to shell out extra money for different devices, etc. Unity’s business model should work for all business models in such a way that they can be paid their dues. Also, the more a game is used the more demand this puts on the developer for upgrades, bug/security fixes, supporting other devices, etc. This demand will translate into demand on Unity, which makes it only fair that Unity gets payed some amount based on installs.