> Neovim is being sold as a VIM refactor, but apparently with no new functionality

Where did you get that idea? When I read the project page (https://github.com/neovim/neovim), two of the explicit goals are:

"Enable the implementation of new/modern user interfaces without any modifications to the core source.

"Improve the extensibility power with a new plugin architecture based on coprocesses. Plugins will be written in any programming language without any explicit support from the editor. "

It is exactly designed to be a playground for new features, but with the goal of them not having to be in the core.

Neovim isn't a simple refactor though. It adds the ability to do many things which aren't currently possible or are extremely difficult to do with the current vim. Sometimes there is no easy way out and you have to put in the work to fix previous limitations.

To a degree, I think these distinctions both support and are a logical consequence of haberman's argument (please correct me if I'm wrong). As computing systems evolves, we learn more about how to address their constraints and how these constraints impact how users choose/desire to interface with their programs. Vim is publicly ~22 years old. During it's maturation, many computing milestones have been met which overcome many of the original assumptions or challenges which prevented us from using some of the interfaces we prefer today. Async (re: MarcWebber) being one such example.

I believe tmux / screen is a beautiful example, given Screen has a lot of black magic and has a very challenging codebase to work with. A lot of the philosophy built into tmux are cumulative learnings deduced from what was done correctly or incorrectly within screen. This materializes as features missing (or implemented with different intent) from tmux that exist in screen -- the philosophy behind sessions is one such big underlying principle change.

I like to think of this as similar to using a genetic algorithm to evolve an overly complex/convoluted solution over a period of time (based on some assumptions which may no longer even be applicable), and then explicitly defining/reconstructing a comparable solution using more deterministic and intentional approaches. The focus of these two development cycles are fundamentally different and perhaps neither wrong (the former attempts to discover novel ways of addressing a problem whose solution is unknown, the latter prunes this known search space).