True, but if there's a will, there's a way.

They can compromise on the process and maybe aim for a medium to long term merge.

I.e. they start aligning coding conventions, standards, etc, and when they're ready 1-2 years from now they pull the switch and re-merge the code bases.

There is one thing that vim provides that the neovim folks have said that they will never provide, and that’s a GUI.

Unless the vim project gives that up (and there will be significant outrage over that), or unless the neovim folks give up that particularly shortsighted stance (having a first-party cross-platform pluggable GUI is a good thing), I do not see a merge ever happening.