Opinions aside, it is objectively an increase in syntactical complexity, and many people who love lisp enjoy its relative syntactical simplicity.

Whatever is gained in exchange for this additional syntactical complexity may not be valued in the same way by everyone.

So that almost certainly explains that reaction.

Yes, and a lot of people recognize the value of having a little bit of syntax in their Lisp.

Yup. Somehow, I prefer[1]:

    (funcall #~s:gi/abc/def/ "Testing abc testing abc")

to the five lines of the equivalent made with macros, function calls, and keyword arguments.

I know all the problems with reader extensions, really. I understand being cautious. But at some point, you gotta wonder: what's the point of the programmable reader when you're unwilling to add programs to it?

[1] Let over Lambda: https://letoverlambda.com/index.cl/guest/chap4.html

I'm reasonably happy with:

  1> (regsub #/abc/ "def" "Testing abc testing abc")
  "Testing def testing def"