Because rewriting or shimming the 10MM lines of code in the libraries one depends on outweighs this one particular benefit of erlang.
> I don't believe it is possible to do real concurrency on the JVM, without rewriting key parts of the JVM
Define "real concurrency".
Multi-threading is doable in any number of languages, but the problems inherent with it are why people move to concurrency.
Concurrency needs to be supported in the language itself.
You'd spend 20 years recreating this in C++ vs 2 weeks learning erlang.
Because rewriting or shimming the 10MM lines of code in the libraries one depends on outweighs this one particular benefit of erlang.
> I don't believe it is possible to do real concurrency on the JVM, without rewriting key parts of the JVM
Define "real concurrency".