> and therefore see no reason to try and coexist with it

This is my main point of disagreement. This is not their decision to make.

> The authors can’t assume you’ve read the manual or understand what the program was designed to do

I think this is correct, but, the principle of least surprise should be paramount here. If the program states something like "migrate all vim-related files to neovim(This will lose all metadata when opened by vim)? y/[N]", I'd concede that neovim wiping data if the user chose Y as acceptable. Yet, they neither display the prompt, nor keep existing data.