Obviously I don't know the inner workings of Unity, but Unreal just has a flat 5% royalty fee after a game makes 1 million USD. Which seems to work fine for them. They also run several games owned by their parent company, which makes tons of money. That was something Unity was going to do until they scrapped the project recently.
Really, if they need more money they could just increase the prices of the previous model, or charge per game purchase. Charging per install is insane and potentially dangerous. It opens the door for Unity to lie about how many installs a game has and illegally steal from developers with no way to hold them accountable, or to include spyware with their software to track installs, or for anyone with a basic understanding of programming to install-bomb any studio into bankruptcy. Unity has kinda walked back on some of these changes, but in a very unclear way that's just led to more confusion. And it's unclear if some of the worst exploits are still possible (especially since Unity is financially incentivized to allow install-bombing and charging pirated games).
It should also be mentioned that their current CEO was the CEO of EA while it was ranked as the worst company in America. And might have done a bit of (extremely illegal) insider trading just before this announcement. So in all likelihood, Unity doesn't need more money, the executives are just being greedy.