People talk as if dynamic scoping was objectively a mistake, but the fact that it works well and is really useful in a complex piece of software like Emacs seems to suggest otherwise.
original opposition to lexical binding in Lisp circles was that lexical would be slower. That turned out to be false.
Emacs Lisp explicitly kept to dynamic binding for everything because it made for simpler overriding of functions deep in, but resulted in lower performance and various other issues, and ultimately most benefit from such shadowing seems to be focus of defadvice and the like.
I can understand why that objection would be raised, because lexical binding is slower in code that is interpreted rather than compiled, compared to (shallow) dynamic binding. Under shallow dynamic binding, there isn't a chained dynamic environment structure. Variables are simply global: every variable is just the value cell of the symbol that names it. The value cell can be integrated directly into the representation of a symbol, and so accessing a variable under interpretation very fast, compared to accessing a lexical variable, which must be looked up in an environment structure.
A rather weak argument when you consider what kind of mechanisms (like a digital clock with working seven-segment display) people have been programming / put together in Conway's Game of Life; to me this does not suggest in any way or manner that GoL could ever be my favored platform to simulate a digital clock (or anything more complex than a glider for that matter). Likewise vacuum cleaners and toothbrushes have likely been made hosts for playing doom, and people accomplish all kinds of stuff like quines and working software in brainf*ck. None of these feats are indicative of the respective platform being suitable or the right tool for a sizable number of programmers.
As someone that used it in languages like Clipper, or Emacs Lisp, and ADL rule in C++ templates, cool to make programming tricks of wonder, a pain to debug when something goes wrong several months later.
Few deny the utility of dynamic-style variables for certain kinds of programming. But it can be helpful to segregate that behavior more carefully than in a language where it is the default.
