The merge will absolutely happen but it will be less of a merge and more of a takeover. iPadOS apps running on MacOS is just the thing to ease you in, eventually at a WWDC down the line the words “Most of our legacy MacOS users are spending most of their time in iPadOS native apps…” and your old versions of Photoshop or whatever will be shifted to be the ones living in the emulation layer like Classic OS9 apps to eventually be removed completely.
If you think I’m wrong, forget your own opinions and prejudices to iPad and walled garden computing and imagine you’re an Apple exec who gets to see earning charts, iPhone, iPad and MacOS. One of these things is not like the others in both usage numbers and profit.