If Apple is forced to allow third party app marketplaces it is clear what will happen. Every major player will make their own marketplace and force you to use it to access their software and whatever other software they have paid to make exclusive. Epic is one of the absolute worst offenders for this sort of behavior. So to everyone claiming "you will still be able to use the curated app store", I guess that's true but only technically.
IOS will not be a better place should Apple be forced to go through with this. This is not being an "Apple apologist". I think we all agree that Apple will happily exploit the people forced to offer services through their platform. But if they are forced to take their hands off the wheel they will lose the power that allowed them to push through some fairly groundbreaking privacy protections, including sign-in with Apple, which I personally deeply appreciate.