IIRC Epic Games internally calculated that for their store the break-even point was around 9%. (They mostly run it as a loss leader at a default 12%, but with tons of giveaways and deals, so that percent can go as low as 0%.) So I think somewhere around 15-18% might feel “fair” to me, trying to take into account the value of the platform.
That is exactly what happens if they can enforce payments: "you don't get to be on our store if you're bypassing this"
But it isn't what is happening if they are staying on the platform's marketplaces and also bypassing payments. There is no "market" effect there.
Not saying I agree with the 30%, but third party app stores exist. That is the market avenue (and no one uses them).
When two entities control essentially the whole "market" for mobile OSes and associated app stores, and use their position to force their app stores on everyone, you no longer have a market. If we just forcibly split Google and Apple into smaller companies with separate app stores then maybe we could see what markets would do.
"There is no market effect"? Why do the market effects disappear if some of the players don't play completely according to the desires of other players? Why couldn't it be that the optimum includes some amount of fee dodging?
Why wonder whats fair when we could let the market decide?
E-feudalism isn't capitalism.
The gatekeepers are governments without democratic representation. Wondering what fair exploitation looks like is choosing a warped perspective.