For the base game, which I think 30% is still more, I think it certainly makes sense.
Because they're providing a complete solution.

For in-app purchases or unlock purchases, whether or not the purchase is in-app, the solution isn't complete, and not worth the 30% they charge on those transactions. It would be trivial for every transaction to have a custom field where you could store an array of what was purchased in in that purchase and have it returned when the transaction was checked. Boom, complete solution. Specifically for in-app purchases if they wanted to take 5% since all they're doing is the job of Stripe and nothing more, then I'd consider that fair.