There are certainly different possibilities of where the line would be drawn. I am not that familiar with vim internals, but I don't consider file management to be part of vim editing model. And we are only talking about editing model (e.g. b being beginning of a word), not the whole editor (plugin system, ui, lsp and treesitter integration, etc). Though it is true that some parts are more tightly related to "vim". There some cases were Neovim already wants to disconnect itself from vim legacy like allowing to scroll viewport without moving the cursor
By "recreating everything" I was also referring to the ecosystem around the editor
But doesn't the ecosystem around the editor depend on the editor being, at its core, Vim? https://neovim.io/doc/user/lua-guide/#lua-guide-api describes "three different layers": basic examples like https://neovim.io/doc/user/lua/#watch-file call into the Nvim layer.
I think the idea's good, but I don't think you could modify Neovim to support it while retaining the ecosystem – not without also modifying much of the ecosystem, at which point it may be easier to do a straight port to a different editor, or construct a compatibility layer (essentially reimplementing enough of Neovim in the new editor that the ecosystem still works).
Add that to a long list of side projects I would like to do "at some point"