There are so many more alternatives for the JVM these days - languages like Scala, Kotlin and Clojure - I wonder why these companies do not invest in the future and still push the old Java around.
I guess rewriting all your code is one thing but rewriting it under the assumption of concurrency is just a bridge too far.
I think you might be thinking that the impediment to removing the GIL is user code. It is not. The impediment is that CPython - the standard Python runtime - was designed around using the GIL, and it's extraordinarily hard to remove it and still meet the existing single-thread performance goals. There are also ancillary issues surrounding user code - particularly modules written in C, I believe - but the runtime itself is the biggest issue.