That's nice but we can all agree that it's not the same thing as a standard builtin syntax that everyone knows and is forced to use.

I can agree it's not the same, but what's the point? A more interesting disagreement is that I wouldn't say it's a downside (though yes, there are tradeoffs). Especially in Current Year when open source is fashionable and pretty much every language has a package manager to make pulling in or swapping out dependencies pretty easy, I don't see the issue. It's also interesting to note that of all the things Clojure did to "fix" shortcomings of past languages with a more opinionated (and often more correct I'll admit) design philosophy that users are forced to use (even when it's not more correct), infix-math-out-of-the-box wasn't one of them. I don't think that specifically really hurt Clojure adoption. (But of course Clojure is reasonably extensible too so it also has a library to get the functionality, though it's more fragile especially around needing spaces because it's not done with reader macros.)

I've brought the library up many times because CL, unlike so many other languages, really lets you extend it. Want a static type system? https://github.com/coalton-lang/coalton/ Want pattern matching? No need to wait for PEP 636, https://github.com/guicho271828/trivia/ If all that keeps someone from trying CL, or from enjoying it as much as they could because of some frustration or another, due to lacking out of the box, chances are it is available through a library. (Or heck, even just tooling setup. Tab completion for symbols and jumping to function definitions are just two great things modern programmers enjoy in all sorts of languages, CL too, if someone doesn't have that setup I wonder if they must enjoy pain.)