Developers have been so steeped in propaganda about "payment processing" that they have no idea what the 30% commission is for.
A court or regulator could force Apple to allow third-party payment processing, side-loading, and third-party app stores, and Apple could change their business model to directly charge for the tools and the SDKs that every developer uses to make native apps. The only way to avoid those costs is the way that already exists today: make a web app.
Right now the payment processing costs have been integrated into the commission, but they could charge whatever the market could bear to license their IP for a royalty, so in effect developers could end up paying more.
There's nothing in law that says Apple can't charge money for their software. That the use of developer tools and SDKs were free on other platforms like the Mac doesn't mean that it's legally required that it be given away for free.