There is a start on VA.gov already happening in this direction. The front-end codebase and backend (Drupal 8) CMS are both fully opensource for VA.gov.
* Frontend - https://github.com/department-of-veterans-affairs/vets-websi...
* Backend - https://github.com/department-of-veterans-affairs/va.gov-cms...
Paul's company is working on this project too (whom I work alongside with but with a different company)!
The U.S. Digital Service is really a big factor in this, see the playbook, specifically play 13:
> If the codebase has not been released under an open source license, explain why.
source: https://playbook.cio.gov/#play13.
So, see how this is now flipped. It is now "hey, you need to open source this by default, and if not, you need to explain why you didn't." and not the other way around!
Update: See Andrew Gunsch's comment in this thread too!