They are not just "merging codebases". They are literally letting it stagnate in all the parts that matter, and all the user-facing parts are iOSified, or literally delegated to iOS with "you can run iOS programs directly now".
And yes. Merging codebases is a bad thing when it's done without any care for one of the paradigms. The one that they don't like, don't understand, and want to go away.
And then they keep adding iOS UIs into MacOS, produce horrible laggy iOS-optimised software for it, and call it a day.
Actions speak louder than words.