This is the perennial argument that IMHO is based on a fallacy. If the vim people suddenly stopped working on vim, it doesn't mean all their effort would go to neovim. People work on what they want to work on in open source. Also the two projects have very different goals/philosophies. The code bases have also gotten pretty different in architecture because neovim did a monstrous refactor. It's open source working as intended that we have both.

Could say the same thing about people working on neovim