Yes, unity costs money to develop and a fee is reasonable. But I think the are a few risks with this model.

How do they track installations? Metrics from steam and other platforms? Connecting to a license server at install time? Or maybe at runtime? I don't know the answers but they all seem to have implications for users regarding privacy and/or offline gaming.

It's also a variable fee to game developers. A single user can install a game on multiple devices despite buying the game once. Similarly, a game can be installed repeatedly over time. This is a financial risk to game development companies. I could see them mitigating this risk in several ways. First, they can pass the fee to the end user. So every install costs the user $0.20. Secondly, they can limit the number of installs per user. You want to install more than 5 times ever? Buy the game again! Thirdly, they could simply shut down the download service after a certain amount of time, making new installations impossible. None of this is good for a gamers.

And what happens to games made by companies that shut down entirely? Today, games remain available through steam, etc. But with this new pricing model, Unity based games will continue to cost money over time. Who pays the bill after the company is gone? This reminds me of Worlds Adrift, a game that used a licensed library. When the developer company shutdown, they were unable to release their server source code because the third party couldn't can't send bills to the open source community. Thus, the servers were destroyed and running the client today (still vailable via steam!) just gives the user an error message about license issues or something. Users paid for a game that they are now unable to use.