This was true for a while, but vim 8 (and some of the last patches of vim 7) added async tasks, channels and built-in JSON encoding/decoding. It's a hassle to have to support two different interfaces, but I suspect most plugin authors are going to do that rather than force their users to use neovim. Some plugins (like neomake) already support both.

Edit: Forgot to mention, many of the improvements in vim 8 were clearly in reaction to Neovim's innovations. Both projects benefit from the other, this isn't a fight to the death.