Vim didn't eat Neovim's lunch. It made its own lunch. Vim8's async feature is a copy only in name. Comparing both async APIs, I'm almost certain that Vim8's was added spitefully. It's just so obtuse compared to Neovim's.
Well, that one is the one people are using. Now that the question among plugin developers is "Vim or Neovim?" which one do you think they will choose? It's totally possible that Neovim will need to adopt Vim's inferior API for compatibility reasons, like they're doing with the VimL<->Lua compatibility layer.
The same question can be asked about Vim 7.4 and Vim 8.0. Which should a plugin developer choose to support? I'm a plugin developer and choose to support any version that has the right features for what I'm accomplishing. For most things, there isn't anything special that needs to be done.