We all know there's a walled garden. Don't like it? Run android. No one is forcing a developer to sell their product through the app store (just like no one forces a business to sell through wal-mart, target, or amazon).
I detest microtransaction/loot box/gambling/pay to win games, and really don't like software subscriptions either. Does that mean I have a right to a perpetual license? Nope. My only recourse is to not play them (which I do for microtransactions), or suck it up (which I do for most software subscriptions).
If they lose, and are going to have to allow other payment systems, Apple will have to build a new way to monetize the app store. That could include charging for reviews and vetting, paid placement in search results, fees for xcode licenses, etc. That's a complex issue to get right for apps from 10 users to 10 million, but I bet they've already got an internal team working on an 'if we lose' strategy. Don't think for a minute that prices will be cut or Apple's revenue will drop.