That depends on your goal. If you just want to log in and work remotely in neovim, sure. But neovim won't provide tmux's feature of being able to reattach to a session in the face of unreliable connections.

That said, most of the time I use tmux is on localhost as a tiling window manager for terminal-based work. In those cases, it's very rare that I use tmux session recovery, so there's merit to simplifying the stack by dropping tmux.

Even if you did adopt the "nvim multiplexer" approach for local work, you could still use it remotely inside of one simple tmux pane. (i.e. ignore all features of tmux except for session management.)

I don't think so. This is why I primarily still use tmux, although I've recently been messing around with i3 and AwesomeWM on my linux box, which is superior to tmux. DTVM seems to be gaining ground, it's just not quite as feature rich as tmux.