Whether or not it's "fair" for Unity to charge per install...
they're being very cagey about how they're going to get that number.
Example: I download a Unity game from itch.io. It's in a zip file. I unzip & click on the .exe. Is that an "install?" I put it on a flash drive and run it on a different laptop. Is that a second "install?" How can they tell? If I'm offline when I do this, how do they even know it exists?
Are Steam, Itch.io, GOG, and Epic going to give them access to the number of installations of Untitled Goose Game next year?
Right now, Unity's info on their counting method is "Just trust us, bro."
...I can't wait for the lawsuit when someone they bill demands they prove how many installations happened for that game.
And that's before we get into, "how are they going to prevent hostile bot installations" - someone who decides they hate a dev, so they rig a bot to a VPN and a cluster of virtual machines to make it look like there's 5,000 new installations. Or 50,000.
Charity bundles are exempt... what counts as a charity bundle? Does that mean any bundle, or are they picking which ones count? (Do they have ANY IDEA AT ALL how many game bundles are active at any given time? Or do they think there are, like, six game distributors on the internet and only two of them run charity bundles?) What about not-charity bundles, like the Humble Trove - those are "pay $15/month for a subscription; get a cluster of high-value games, plus keep access to a swarm of over 50 small indie games as long as you're subscribed."
How are they going to figure out whether a game has made more than the threshold? AFAIK, financial records of gaming distribution sites are not open to the public. (Has Untitled Goose Game made more than $200,000 in the last year? How would Unity know?)
They're dodging a lot of very basic questions about how they'll get the numbers they plan to use to invoice people.
Even if they had that - this is the end of Unity as the default "not sure what engine to use? Here, this is free and fairly easy to learn." Because shifting from "free forever" to "maybe we'll charge you some day, some amount that we'll decide, based on numbers that maybe we made up but we're not going to tell you any details," means it no longer works for people getting started in the industry.
(Right now, they're saying they'll charge up to 20 cents per installation. What prevents them from deciding to change that to $1 per installation next year?)