If I get payed 40 cents every time it is opened this isn’t a problem. He can open it as many times as he wants. I’ll happily pay the 20c and keep the rest as profits. If my income is proportional to usage and my costs are proportional to usage there is no problem. I don’t see why this could not hold for games or for PDFs?
The bottom line: if somehow you’ve made a game and it is installed a lot, but you don’t make enough money off of that such that you can’t pay your suppliers then you’ve just failed at commerce.
A friend of mine failed at commerce once. She had a clothing store. In the clothing business you’ve got seasons. So typically shop owners buy a whole lot of clothes in bulk for the entire season. Her shop didn’t survive the economic down turn of 2008/2009. So she was left with huge amounts of clothes and an enormous bill to pay, which she had to default on. Unity’s business model is extremely mild compared to that industry. I also still fail to see how it is not fair.