My guess is that as a fork of Vim they’re intending to replace Vim and therefore see no reason to try and coexist with it. They would probably say something like “why are you trying to use Neovim and vim at the same time?”

Whether or not that’s a good design is definitely discussion-worthy.

I’m not sure I agree about the concept that programs promise not to fuck up your data without your consent. Programs can’t really know what you are consenting to. The authors can’t assume you’ve read the manual or understand what the program was designed to do.

I think there’s a decent argument to be made that persistent undo files are more akin to metadata, similar to Microsoft Word recovery files or search indexes, but I haven’t made that argument because the author’s frustration on the issue seems logical and valid. It seems like it’s still not meant to be a data storage or backup mechanism: you wouldn’t share it in an email or check it into source control.