Immutability is rigidity. Of course, it comes with tremendous benefits as well, but as an effort latency vs bandwidth thing, there's significant latency with using Lisp intellectually. Would you use it for shell scripting or a simple glue language? I wouldn't, and I love Lisp.
Lisp is the language of the future. it's always been the language of the future, for nearly 60 years now. It always will be the language of the future. We should think hard about why Lisp has both never become mainstream and never died out. But the fact that it never become mainstream, especially if you exclude conspiracy theory and "most programmers are idiots unworthy of our wisdom", is telling.
Rigidity may not be the reason Lisp never broke out. But there is some human factors reason that's worth understanding.
But Lisp is no more inherently immutable than any other language—depending, I guess, on what you mean by 'immutability'. The runtime is certainly highly mutable (although I guess that would usually be called dynamic). It's good practice to use lots of immutable data structures, but that's hardly required, nor an inherent feature of the language.
