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

The implication that we can make all software FOSS and have nothing else about the world change is a textbook example of putting the cart before the horse. It's like asking "what if everyone became vegan, who would pay the cattle ranchers?"

The world FOSS strives for, the world where it is the norm, has a fundamentally different economy from our own.

It's not a valid thought experiment to ask "what if all software was FOSS (but nothing else changed)?" because that creates a hypothetical world that has a fallacy at its core. A world where entire social movements can blink in and out of power without regards for sociological and historical factors is a world unconstrained by logic as we understand it. The correct framing should be: "what would our world have to change to enable FOSS to be the norm?"

The distinction is subtle, but cuts to the core of the contention betweem movements aiming to change the world in radical ways and their detractors offering criticism that boils down to "but the future you propose doesn't integrate seamlessly into the present state of affairs."

We all want change, we just don't want it to change things.