I'm not entirely cynical, people generally are very open to be generous with one another and collaborate for a common good, but up to a point.
Currently people spend the majority of their hours doing relatively hard work for the collective's benefit (kinda). Exactly because capitalism makes selfishness into selflessness (very kinda). Also people are relatively civilized to one another thanks to the considerable latent force of the state's monopoly on violence.
People will be nice to each other when it doesn't cost them much and/or when the opposite costs them dearly. But will they work as hard as now for each other just to be nice? Will they not harm each other when there are no significant consequences and something to gain?
A fair free market is far from a natural phenomenon, it needs to be protected and maintained by some external force. If you let things unfold naturally, what you get is kings, and many layers of dominating hierarchy underneath, exploiting the masses, which exactly what we had the whole time.
I suppose that the post-scarcity idea is that people neither need to work hard, nor have significant reasons to harm, if they have everything they want. Sure, let's talk if we ever get there, but until then we have other problems to deal with.
PS: Don't forget that people are able to do FOSS because they have well paid jobs that don't completely drain them of their energy. For others, getting the reputation and/or experience for a better job is the incentive. There's a very different social infrastructure making that work, FOSS doesn't sustain itself, not even close. But yes, it does prove that when people's needs are covered, some of them will do great things for everyone without much incentive, but usually not enough to cover everyone's wants.