I'm literally writing a blog post about this now (I'm building an objects system from scratch using only FP techniques in JavaScript... slightly eclectic ;-) ). The very interesting thing about this exercise is that immutability solves a huge host of problems in OO systems. As a very small example, even the diamond inheritance issue just goes away because nothing is ever updated. If B and C both have the base class A, you can safely instantiate A twice because it doesn't matter which one you use.

I'm about half way done. When I'm done, I'll post it on HN, but if you are interested here is what I've got so far: https://github.com/ygt-mikekchar/oojs/blob/master/oojs.org If anyone ends up reading it, I'm very happy to receive criticism, so feel free to leave an issue.

Sure, you can simulate immutability in Ruby or Python but it's not real immutability. It's just shallow freezing. The real point, however, is that immutability is not idiomatic in OOP languages. Objects are intended to store state and have it modified. It's not what a language can be made to do that matters. It's what is idiomatic.

And yet 99% of code written in Java, Ruby, <Generic OOP language here> is based on mutable objects.

Its not about what is possible, we're all familiar with the turing completeness of everything. It's about what is: the default + easy + encouraged in the ecosystem.