iOS won’t ever be a development platform. That would necessitate bringing down the AppStore walled garden, which would be a very stupid move from Apple.
Not sure why you say this. There are already various kinds of developer tools on iOS (see Codea, for example). They don't bring down the App Store walls.
An Xcode for iOS would be designed to make it straightforward to develop iOS apps and then sign them and upload them to the App Store when you're ready.
Sure, you can have toys like Swift Playgrounds in the AppStore, but real application development takes more than just Xcode. E.g. you need a shell and various command line tools, like otool, lipo, nm.
