When he talks about customers and stakeholders, he is talking about people who have bought in to that design. It is very easy to support his position here. Rich knows exactly how he wants to program and the man is a visionary of data-driven programming and thinking. If you don't like that vision, maybe don't use Clojure and find a different lisp.
Great design is a very foreign idea to a lot of mainstream software developers - most of them, sooner or later, go for the "big rewrite" because they didn't get the design right to start with. Things like Python 2 -> 3 spring to mind (breaking changes to print! whoever thought that was a good idea didn't respect the language users). With that rational, he is promising not to do exactly what the Python people did.
> Something that is often very hard to understand (it took me years to do so). Is that maintaining a language is insanely hard. Everything has a cost. Let me give a good example: A few years back someone submitted a patch that improved the error messages in Clojure. Worked great, just a single extra "if" and a message. It was committed to master. Then people's code got way slower. Why? Well this was a often used function and that single if blew the JVM inline budget causing that function to never be inlined. And that improvement had to he yanked out.