That's not my experience. I'd say that people know what the CL spec currently is, love its stability, and might fear what it could become after an "upgrade".
Anybody can write something like CL21, for example, with one's own style and notations. That's why it is frowned upon to do so and to claim that "this" library is how "the" new Lisp should be. Part of growing in Lisp (and programming) is to learn how to work with a language, not against it, and avoid fixing what is not broken.
However, I trust Common Lisp compiler developers and vendors to know how to make backward-compatible, useful changes: current common lisp environments have way more facilities and building blocks than the bare minimum required. They also know the pain points that currently exist in the standard in a way that goes beyond aesthetic things.
I find interesting your remark that "anybody can write something like CL21". (Though, not all problems can be solved with a library, but let's stick to this side). In most language, you can't do this kind of customization. In Lisp you can, but anyone else can too, and the result is that after 25 years there is still no standard way to write a hash literal, for example, or to concatenate strings in a non-verbose way. It is difficult to claim that these are good things.
UIOP:STRCAT. UIOP is present on every contemporary Lisp image as a part of ASDF.
But, if UIOP:STRCAT is such a "standardized" solution as you claim, then I believe it should become part of a (hypothetical updated) Standard, not a second-class citizen behind a member of the ancient aristocracy like Lord CONCATENATE.
What I mean is that I think there should be some kind of mechanism so that standardized solutions emerge (from widely used libraries for example) and become part of the Standard. Like it happily happens in many other domains.
There are people whose jobs is to work on that.
When you write "it should become part of a standard", you use the passive voice: who is going to do it?
In CL, the effort is focused on things that matter to each individual, or each company that uses it, and sometimes a (de-facto) standard library comes to life.
A serious effort would probably benefit from talking to the C++ people. Since my knowledge of C++ is basically stuck at C++98 + a few Boost utils, my outside perspective is that C++0x and eventually C++11-C++17 were just standardizing what were already halfway de-facto standards in Boost (halfway because you could get a lot of questionable looks for including Boost from some parts of the community, and the limitations of C++ meant some things didn't look or debug as nice as if there were compiler support). Still, with the standard the syntax has grown quite a bit and some of it can be more pleasing than the Boost macros. Did it break anyone? Probably, but at least compilers let them compile with old standards. It'd be a good conversation with C++ committee people especially to see what it took to finally update such an old spec. A CL update would surely need a similar feature of running under the old spec, since e.g. if you're adding literal hash table support you're almost surely going to break someone who was using that syntax choice for something else.
As an aging developer I also detest environments that break things every other day, but I have realized that on the other hand system that never move have indeed a tendency to wither, usually in comfortable niches, while the flow of things moves elsewhere.
Amen!
This is not necessarily bad, especially in the context of education. It is well known for ages that a bit of mystery and sacredness makes rather boring moral education much more emotionally charged.
There are no active Common Lisp based courses, as far as I know, the last one was that one of the Alegro team, but it was rather an attempt of promotion and has been done without "religious vigor" unlike the famous CS61A by prof. Brian Harvey, which could be viewed as almost sectarian, especially the very first version from 2008. Similar course could be easily made based on Common Lisp, which includes more advanced concepts in CLOS.
At least some zeal is good for education. CS61A and the original SICP course by the Wizards are good and still relevant examples.