I keep hearing that Apple's direction of travel is to sunset MacOS for iOS. If that's the case, Apple will need to provide developers a path to develop for iOS on iOS. So there will be a need for an "expert / developer mode".
I hear a lot of people say that too, but Apple itself has repeatedly said that iOS and macOS serve different purposes and they will remain separate products.
What we do see Apple doing is consolidating some aspects of iOS and macOS development. That makes sense since iOS was originally built on the OS X kernel, but then allowed to diverge.
Would they have to? I mean, yes, that would be nice, but for example people developing for game consoles don't develop on the console itself, but rather using a separate development system.