I hate to be flippant, but that's a well-known business model called "contracting". It's possible, absolutely, but it requires a substantial investment of time and effort in business concerns on top of the actual software development. In practice, most leaders of contracting businesses don't have time to write code, and most contracting businesses end up deciding they need to release closed-source extensions.

How would e.g. Mozilla fit in that view?

As far as I know Mozilla doesn't sell any software licenses. They were almost entirely subsidized by Netscape from 1998 to 2003, then by Google from 2005 to 2014, and by a variety of search engines competing for Firefox default status since then. That kind of "cash cow" model gives you a lot more organizational freedom than software licensing.

Then you've re-invented being a software vendor that can't even pay it's developers reliably?

Why not reliably?

And why would you compare it to a regular software "vendor" when it produces free open software for 99.9% of the people, and non-free open software for 0.1% which are large corporations?