Unity already charges money once you hit a certain revenue from your game, it's only free if you don't get a lot of sales.

More importantly, according to the article, when questioned it seems Unity hasn't given any proper thought into this model.

If a user installs the game and then uninstalls and reinstalls, it counts as 2 installs that must be paid for. Not only is that unfair, it can lead to abuse. Angry with a change the developer made? Uninstall and reinstall 30 times (automate it) and you will actually cost the Dev money.

What about pirated copies? Unity will still "phone home" and the result will be a developer paying for 1mil installs that he earned nothing from.

What if your game is free to play?

There were some other issues like that mentioned too (in the twitter post in the article).