It could. But still, someone has to develop iOS itself. And someone has to write the server back-ends that all those apps depend on. And the cloud infrastructure that those back-ends run within.
By definition there will always be something out there that can't be developed on a locked-down platform. So there will always be an open platform, even if Apple decided one day (again, unlikely I think) to recuse themselves from that market.
Apple always has the option of fusing their Macs and selling blown ones externally, and then writing their OS to turn off parts of the OS that they use but think you don't need. They haven't done that, but it is always possible that they could.
Sure they could. They could basically fork macOS and maintain an entirely separate version just for developing iOS, iPadOS, WatchOS, and macOS. I don't know why in the world they would do that.
This whole "issue" is just Apple trying to get people to put their software in the Mac App Store. Maybe a day will come where Apple try and lock down macOS to the level that they have locked down iOS... but I just don't see it happening. Apple will always need developers to develop software for Apple products.
Maybe they will drop a nuclear bomb on themselves. Anything is theoretically possible.
By definition there will always be something out there that can't be developed on a locked-down platform. So there will always be an open platform, even if Apple decided one day (again, unlikely I think) to recuse themselves from that market.