It never really feels like Vim, more like a minefield of unknown shortcuts with a layer of Vim bindings on top. But I get why some people like it, if you can't stand modal editing then Emacs is a great choice.
1. Language is more extensible because of macros so dealing with editor state can be hidden.
2. Language is more dynamic with advice and eval-after-load and such is extensions can play well together
3. For all it’s difficulties, having buffer-local variables and dynamic scope seems to make writing extensions easier
As far as I can tell, making Emacs cooperate with neovim would require shoehorning the editor model of one editor into that of the other, likely causing many extensions and expected behaviours to break.
There is, I think, a difference between making a new application that can use neovim for editing some bits and making an old (or, indeed, very old) application switch to using neovim for editing.
I'm sure a lot of this can be fixed by changing some configuration options, but I don't see a reason to change the editor just for the sake of it. Emacs is awesome in some ways, but it's not my cup of tea.
:set virtualedit=all