"Do open source stuff" is very different from "being open source companies".
I actually do think that the 'sweet spot' is to use open source as infrastructure, and make money with something proprietary, but that's not something that makes purists happy.
Well, you wrote "doing open source stuff", not me :)
I agree that it doesn't make purists happy, but frankly, I don't find it particularly important. As long as proprietary software exists, whether companies are separated between open and closed or hybrid isn't particularly relevant, in my opinion.
The dichotomy is this in any event: it's easy enough to work on free infrastructure stuff if you make your money elsewhere, but it's difficult to work on free infrastructure stuff as your business.
What you want to look at, though, are established companies with customers doing open source stuff, like Redhat.
They do exist, but it's not easy.