Take away the apps/services not created by Apple and the value of the platform drops considerably. Whole market is in a big catch 22 position. It is nearly impossible to compete and create a new smartphone platform, because even if a new player creates superior products compared to Apple, they don't have all the apps. They can't create the whole ecosystem by themselves, as the costs would be insane. Software developers also don't have much choice. There are two platforms that control all the market share. They have to accept the rules given to them or leave the industry (one they helped create).
Saying things like that every developer agrees to the App Store rules before releasing their applications is a major oversimplification of a complex situation. Apple can change the rules and they might not be the same at the time when developer makes an major investment to develop an app and when they release it. And rules might change again after they release it.
Apple did not create this whole virtual ecosystem, just the platform on which it is build upon. And this ecosystem has outgrown Apple. It's got so big, that Apple's contribution is only a part of it. Apple has been generously repaid for their efforts, and will continue to be so in the future. But maybe they shouldn't have a complete control over a virtual ecosystem this big, where their investments no longer represent the majority of it's value.