If your revenue is above $100.000 the last 12 months, you need a professional license. Which you pay for. The “free for smaller games” is what allowed Unity to gain it’s current foothold in the market. This install fee will be in addition to that. And for all games, including older games or games made on older versions of unity.
It takes years to develop a game, and Unity announced this pretty recently (September 12). If you had a plan that would be profitable with ads or microtransactions and you and your team spent years making it, you’d suddenly might not have a business model any more. And for games already released, it might not be profitable keeping it up any more. Unless you have a way to predict the future, that point is completely moot. If you started developing a new game the last … 5 days, sure. But then you’d probably pick a different engine that doesn’t have such a requirement.