Since there is only one app store for iPhones, almost every app vendor is willing to conform to Apple's guidelines (which are often pro-privacy and protect the user). Otherwise they lose out on a significant market share with a lot of spending power.
smaller party to make a competitor on the official store
Sure, they will pop up. But Facebook, Microsoft, and Google will start iOS app stores and app developers will go to their app stores because of network effects.
You've just described why these changes are good. I feel like the word "willing" in your statement is carrying a lot of weight.
Apple forces developers to publish from Apple devices, spend $100 a year for a developer account, give up 15-30% of any revenue generated from that app, use WebKit, etc.
That is not at all what I describe as restrictions that lead to "willing" app vendors.
They might try, but it would be a lot harder than you imagine.
As apposed to today where you can’t get it at all if apple and the app disagree about anything?
I know you are thinking of another large enough player you don’t trust as much forcing their store as the only avenue for an app, but it’s hard to imagine how that wouldn’t provide large incentives for a smaller party to make a competitor on the official store.