It's normal for a engine to have licensing requirements, but those are laid out up front and will typically be defined based on income. So like a pretty common thing would be something approximately like free for the first $10K earned, then 10% for up to $100K, and then 30% for everything past $100K. Importantly though, that's NOT a subscription, it's the terms of the license you agree to in order to use the software, you aren't paying a fee based on time, but rather based on money earned. You can choose to back out of the license at any time, you just need to stop selling the software, and as long as you keep paying the engine developer their cut you can keep on selling copies. Further the terms of the license are what they are when you download the library/framework, and they can't be retroactively changed. If tomorrow they decide to start charging you based on total downloads, you can choose to keep distributing the previous version under the previous license terms based on profits.

Unity on the other hand, has done two things. First they require an ongoing subscription, so if you stop paying for your subscription, technically you're no long allowed to sell your game. Secondly, and much more controversially, they're defining the license based on installs rather than based on earnings, which is tying your debt to actions of your customers rather than your own, which is a very precarious position to be in.

This whole thing reminds me of the D&D shenanigans a few months back where Hasbro tried to retroactively re-define the terms of their "open source" license, and the TTRPG community collectively told Hasbro where they could stick their new license. There are a LOT of parallels here.