Most "software development" is internal software that exists simply to solve some organization's bespoke issues, and is not open to the public - FLOSS vs. proprietary is a non-issue there. As far as off-the-shelf, non-internal stuff goes, software maintenance is actually a bigger issue than software development per se, and it's entirely appropriate to say "I will not be putting any effort into maintaining this unless you pay me some real $$$ for the trouble." So, I'm not even sure that we're disagreeing about anything of relevance - except inasmuch as "being able to work on the project - or to sponsor work on the project by third-parties - no matter what" is often very important!