But the python source code's available to build, and anyway, quite a bit of the python in VFX is run through embedded interpreters in Maya, Nuke, Houdini or Kat

But the python source code's available to build, and anyway, quite a bit of the python in VFX is run through embedded interpreters in Maya, Nuke, Houdini or Katana which means they can keep using an old version indefinitely.