I think this is looking at the problem wrong. The problem is that implementing positive change in these systems is impossible for reasons far outside the control of any product designer or developer currently on the team.
This software is old, has byzantine requirements, probably cut costs all over the place, and conceived in a board room without the benefit of an adequate development lifecycle or stakeholders advocating for the users.
It probably takes 3 months to move a button around, and instead of moving that button executives are having them push a feature that earns a few more million, or a feature that the customers want more then a UX improvement.