The argument is that developers have a duty not to arbitrarily delete data on your system that you may have relied on that was produced by another program just because they’d like to replace it with something in their own format.

I don’t care if you’re volunteering your time; if you build something whose implied purpose and success depends on other people using it, you emphatically have a duty to those people not to silently destroy stuff that was there before your software was.

It's worth pointing out that vim is not "another program" as far as Neovim is concerned. It's an earlier version of the same program. Neovim is a fork of vim, not a totally different editor that happens to share part of a name.