If we get a lawsuit that says “you can’t take a cut of every transaction” they’ll just charge a percentage of total revenue and audit developers, or find some other way of getting their cut. It is highly unlikely a path through lawmakers or courts is going to bring down apple and google’s cut. They’re running a platform and they’ve determined a price to be on that platform. They just happened to get that price through IAP, but they have many alternatives.
One example would be access to last mile internet cables and some insurances where profits are tightly regulated.
Lawmakers and regulartors could explicitly tell them what to charge (e.g. "at most x€ per gigabyte downloaded from the app store, at most x€ per store listing,..., No additional charges are allowed")
However, it's a very heavy handed measure and I'm not sure it will happen with mobile phone plattforms.
I am pessimistic of a legal path to reducing the 30% take. You would have to get a court or lawmaker agree it is onerous, and I think reasonable people can disagree on whether that cut is indeed onerous.