I had a client with a proprietary programming language for data warehouse management, they had a very hard time hiring people who wanted to work on that stack from a career progression perspective.

I imagine, at that scale, that opensourcing is a big win because you can have a proprietary stack, but still hire people that already know it, or won't be reluctant to invest a few years of their life into working on it. You also don't have to pay as much for educating new hires as you would for a totally closed proprietary stack, and new people faster become productive. Hiring contractors that work with that stack on a short notice also becomes easier. For a large co, these kinds of benefits might be more than enough to justify cleaning up the code enough to opensource it.