They are providing a game engine to make the game. If that's not worth it, switch. They don't own your C# code or assets, so they can be moved. For a lot of devs, it provides a lot of value. I understand for F2P, low ARPU games this pricing scheme doesn't work and people should complain until Unity fixes it.

Unity has ~7,000 employees, and is losing money. They need to a way to monetize, and they are clumsily trying to target their most successful users to pay more. Price increase aren't rent seeking.