The really big bruhaha came from them doing it retrospectivelly (the legality of which is yet to be clarified and likely depends on jurisdiction) which means games made on top of Unity and shipped would also have to start paying this install fee (even though the version of Unity with which the game was devloped and shipped had no such conditions in their Terms Of Service so the game makers never agreed to these new conditions).

Theoretically if found legal this could not just kill certain business models in the game development community but even bankrupt companies (especially for games distributed free and funded by ads, which are quite common in the mobile space).

Now, maybe, hopefully, such retroactive changes to the pricing will be found illegal in the applicable jurisdictions, but in some it might require a quite expensive legal fight to clarify it and meanwhile many gamedev companies working with Unity run a huge business risk if they ship their products with it.