It’s not the fact that Unity wants to make more money from its engine that has gotten people so up in arms about it, it’s the way it was structured.
If they simply took a percent of revenue, everyone would have shrugged, you might have had some indies complaining, and the world would have kept on turning. Instead, they made a couple of compounding missteps. An install based fee creates a massive amount of uncertainty about how much you will owe at the end of the month, creating potential cash flow issues. You are at the whims of Unity in terms of how they calculate it, and praying that they properly handle all the edge cases and potential abuses, despite all their incentives giving them plenty reasons not to care that much. And then on top of all of that they made it retroactive to all existing games on the market using their engine. Companies had a very reasonable expectation that they could rely on the terms their games were released under. All that trust has now been upended. Unity has shown they are willing to change terms retroactively with little notice. Now when evaluating Unity as an engine you will have to take into account not only the current fee structure, but also the risk that they will add additional fees retroactively that have the potential to completely disrupt your business model.