Google are horrendous (business partner), but at least you can publish and install .apk files without a store. Whether that's a sustainable business model is another thing entirely, but at least you can do it. Not even going to talk about Apple since that's just a more extreme example of the same problem.

From my POV, if you have a business and you rely entirely (or for the majority) of your income from these platforms, then your business is not sustainable and is not competitive. You're playing at a rigged game and you will most likely lose.

I stopped using 3rd party game engines for the same reason. I want to control my source and the potential income I get (if any). A lot of people won't agree with that. That's fine. Google and Apple's duopoly and their respective ecosystems are not good for the future of software. People have to take a stand at some point. These companies want to control every part of the tech stack (and our lives to extent), you should fight against that. We should all fight against that.