The other side of it is if someone has agreed to the new terms and released a game, and that game is pirated, Unity has no way of knowing what percentage of installs are legitimate, then there are purchasers who upgrade their computer and reinstall the game under the original license, and those who bought it but never installed it (look at my stem library if you don't think that happens!)

There is no way to calculate the number of legit installs. You can get close with the game company's sales data (but Unity doesn't have that)

So a games company could wait to get sued, then go to court and ask for Unity to show how they calculated the number of genuine installations

Unity will not be able to show a working method of calculating that as there is none