Anti-competition can cover many areas, and exclusive titles for no good reason is in my opinion also anti-competitive. As you have correctly pointed out, games for PC could launch in any platforms, but by paying or giving incentive to game developers so that you can keep a title to your store is basically to give yourself an edge. Which I feel is a form of anti competition. Competition means everyone sells the same thing, so you differentiate by either marketing, or price, etc... In this case, EPIC is paying game developers and asking for exclusivity to sell on their store. Apple and Google is paying for the development/ maintenance of iOS and Android, which no developers are paying for. So I think it is fair for app developers to contribute to Google/ Apple, because their store is drawing in the crowd for the app developer to sell. As to how much, whatever % they quote, they will not be able to please everyone all the time.
Also I disagree that Google and Apple are "stealing" money from game developers. The developers agreed to the T&C. Nobody is pointing a gun to their head. Developers ain't stupid as well. If they know there are benefits to be in the store, they will then agree to Apple/ Google's T&C.
If I need to make it clearer, imagine you spent your money and effort to open a store. After some time, it is very successful and people are queuing to buy things from your store. Someone then comes in, open up a booth in your shop to ride on the traffic so that they get the opportunity to sell their wares. What would you do? Let them continue for free, chase them out or charge them rental? People appear self righteous, criticizing others about what they think is wrong. But when it happens to you, not everyone will do it differently from who you are criticizing. You don't have to answer here because you yourself will know the answer.