macOS won't be replaced by iOS. tvOS and HomePod don't disappear because iPhones sell more units either. And no, I wouldn't be happy with just an iPad or whatnot, even if it were possible developing on it, which btw, it isn't. Arbitrary code executions is prohibited by the app store guidelines. Playgrounds is unique, but is far from a full development platform. It's a learning and experimentation tool. Full Xcode is unlikely and all other development tools an impossibility on the platform, unless things change significantly. And without a Terminal CLI interface pointless.
It's impossible as it stands to develop software on iOS.
Developing iOS software on non macOS operating systems even is a bit of a workaround, and I think a Mac is ultimately required in the end for signing the code, though there may be ways around that these days with things like Xamaring for using MS tools to build for iOS.
But I wouldn't be fully happy on any other platform but macOS.
Linux or BSD if I wasn't developing for macOS or iOS would be fine as well. Windows only if it's a must. A mobile OS; Never, and currently impossible anyway.
