Immutability has worked out really well for a lot of real-world systems built with Erlang.
The main (and close to only) user of Erlang was Ericsson, and here is what the Wikipedia has to say about it [1]:
"Shortly thereafter, Erlang was banned within Ericsson Radio Systems for new products, citing a preference for non-proprietary languages. The ban caused Armstrong and others to leave Ericsson"
Not exactly a strong endorsement for Erlang.
[1] http://en.wikipedia.org/wiki/Erlang_(programming_language)
"The ban caused Armstrong and others to leave Ericsson.[6] The implementation was open sourced at the end of the year (1998).[3] The ban at Ericsson was eventually lifted, and Armstrong was re-hired by Ericsson in 2004.[6]"
It is really hard to imagine that you made that excerpt in good faith. Read the very next sentence in the article.
Maybe, but concurrency based on mutability seems to have won this war in the real world these past thirty years.
Immutable systems might be easier to design in theory but we haven't seen this proven in the battle field yet, and the fact that most of the concurrent code today runs on mutable structures shows that it can be done.