Did Jython even ever supported all non pure python libraries? The biggest moat of cpython is huge ecosystem of native libraries such as numpy, scipy, scikit, pillow, pandas, opencv, pytorch, tensorflow etc. This is the reason why PyPy is not mainstream. Without all those libraries and ecosystem around it python wouldn't be that useful and for the same reason Jython wouldn't be that useful even if supporting python3 and having the fastest JVM.