[gruseom]

And lexical variable binding comes with significant extensibility downsides when misused.

It sure does. Consider the draconian module system in Node.js that wraps all your code inside a lambda sandwich, shutting it away completely, and then forcing you to petition for access to it on a case-by-case basis. (The justification for this apparently being nothing more sophisticated than "But encapsulation is good.")

This is the programming equivalent of bureaucracy stifling innovation. It is particularly disastrous for writing new tools, which of course is an area where Emacs shines.

If something is not easy to extend in ways that were not intended, it is not easy to extend.