I work in, love, and use almost exclusively open source software and I don't agree with your description. We wouldn't be anywhere if it was only passion projects. For me, I like open source because it avoids wasted effort: everyone works to push the state of the world forward, instead of hoarding their own work for their own profits. It's simply the best way to make software. But that doesn't require doing things only because it's fun and meaningful. Doing work on open source software because you can get paid to do it is a perfectly valid reason to work on open source software.