Not directly. Every "extensions" listed above cannot be implemented as user libraries, that's why these back and forth process between implementation extensions and portability user libraries need to happen to reach a de-facto standard. Otherwise the user libraries become de-facto standard themselves, like ASDF the build system, Trivia the pattern matcher...

But I think the power dynamics is crucial. In the Lisp ecosystem the users have great power, (and there are dozens of implementations,) so it's not like one or two implementation decides what everyone is gonna use and everyone just lives with it.

I don’t know that it’s impossible without that. It’s certainly easier with it.