The OSes have been the same since forever. It's just the same macOS kernel, filesystem, and basic userland with different UI and a few different core libs (given the different form factor and hardware features needed to hook to).

Most of the same people that fear the convergence also ask for "iPadOS" and "iOS" to be more like the Mac (add mouse support, multitask like the Mac, and so on).

So what they do fear is the mythical "dumbing down" of Mac, as in the outcry when Logic got a different skin, and everybody losts their minds as if it's now "just Garageband Pro" (meanwhile it has never been more powerful). Same for FCP (where a few features were lost due to the rewrite, not for any dumbing down).