This verdict does NOT state that an app store cannot charge 30%. The verdict states that Google "engaged in anticompetitive conduct". It is, and should be, possible to charge whichever margin you feel like, for any product. As long as you don't misuse a monopoly or in other ways take part in anticompetitive behaviour, to artificially inflate it. If you are able to charge 99% in open competition, noone is to blame but the competitors.Simple, just make a law that any App Store is not allowed to collect more than 10%. Should be plenty to cover infrastructure costs and moderation.
If you DO have a monopoly, then you will be under scrutiny not to inflate margins. But Google's problem here was that they acted like they are open to competitors, but behind the scenes they made shady deals to push out competitors. Conversely, having a 5% margin does not somehow allow you "engage in anticompetitive conduct". It's about the behaviour, not about the margin. And anyone believing that all of this will end up in lower prices for the consumers, are hallucinating.