Sure . Just make the rules consiostant across the baord. Limit any app store to the same percentage; not just Apple and Google.If Apple think it’s not anticompetitive, they can search for a new own world, have their own countries and citizens there, with their own laws, and keep insulting, dictating and pursuing devs there. But as long they keep doing business here on the earth, they will have to follow the rules and laws countries set up, simple as that. I’m confident they will lose that fight badly.
Of course, that means Apple and Google will need to find other sources of revenue, such as upping developer fees, charegs to review and host an app, etc. They could charge those upfront, which of course would make ut harder for small devs and those with free apps.
Allow other app stores? Sure, but you'll still need the app signed by Apple, for a fee; and a review and fee for each revision. Annually as well to be sure it hasn't changed.
Is 30% to high? Maybe, but, it's a lot less than what it used to be in the day of distributors and physical media, and make the barrier to entry for a new dev much lower. if it is lowered it doesn't necessarily mean devs will get more money with no changes in other fees. Small devs and those with free apps could get forced out of the market.