I suspect the OP is trolling or phishing for material for his own post or essay. Why else would you post such questions in an open-source community? :D

"FOSS doesn't seem sustainable. Everyone can contribute, but how do they make a living?"

Your concerns about the sustainability of FOSS suggest that most contributors must be sidelining it for other income. However, major corporations have built their entire business models on supporting open-source software. Companies like Red Hat are testament to the economic viability of FOSS. Additionally, large tech firms like Google and IBM heavily invest in open-source, further ensuring its sustainability.

"It seems like most FOSS I’ve seen is a free, buggy, alternative to mainstream software."

You label FOSS as a 'buggy alternative,' but it's crucial to recognize that the vast majority of software around the world relies on open-source modules, functions, and frameworks. This isn't a coincidence; it's a result of rigorous maintenance and improvement by a global community of developers. Open source doesn't just run 'some' of the internet; it's the backbone of almost all modern digital infrastructure.

"What if every software became FOSS? Who would put in the free labor?"

Wondering who will contribute the labor misses the point that contributions to FOSS are often mutually beneficial. Developers gain career benefits, improve their skills, and sometimes even receive direct compensation. It's not 'free labor'; it's a collaborative economy of shared resources and mutual benefits.

"Would it simply be that everyone provides for each other? Everyone pitches in? What about people who have bills to pay?"

Finally, your concern about financial sustainability in a FOSS world seems to overlook the multiple ways people monetize their contributions, either directly or indirectly. Besides, the value of open-source contributions isn't merely monetary; it includes invaluable intangible benefits like skill development, community building, and personal freedom.

The questions you're posing have been practically answered by the existing and thriving open-source ecosystem. It's not just an idealistic notion; it's a proven, sustainable, and indispensable aspect of the global software landscape.