What arguments to use to switch from graphical design entry (HDL)?
My question is therefore for seasoned EEs how they were if ever persuaded to use plain-text entry?


I'm not a seasoned EE, not by a long shot. But I do have VHDL experience and one of my jobs consisted of using a combination of VHDL and Python, much like you describe in your question. GUI-style development was simply not allowed. It took some time to get used to, but I never went back.

The testability of one's VHDL improves by so much when not using the GUI that I now consider working without the only 'proper' way of developing it.

Of-course, when going the text-only route, make sure you provide your developers with a text-editor they feel comfortable with. Notepad is too limited and advanced editors like Vi or Emacs can be considered a too big step if combined with dropping GUI-style development.


  What kind of arguments should I use?


That will mostly depend on the company culture. Focus on issues they're currently having which would be solved, mitigated or otherwise made less problematic when switching to text-only. Testability, maintainability and the ability for more flexibility during prototyping come to mind. Don't forget the extra advantages in version control either.

Keep in mind switching from one style to another company-wide is a big thing. It won't be easy and you'll need support of a lot of people to get it working. Otherwise it won't work as well as expected (people 'cheating' the system by doing it their way anyway) and it's one gigantic waste of time and effort.