You seem to be under the impression that everything that's being "produced" right now is of actual value and must be kept up or replaced under a non-capitalistic system. I'd argue the contrary. There is so much braindead wasted labor being performed and energy wasted in the current system that would be completely freed up if our main economic goal were to change from "growth and competition at all cost" to "ensure a good life for everyone". At the same time, our ever increasing ability to automate work and solar energy becoming incredibly cheap means that less and less of the necessary production actually requires human labor.
Add to that that most people like to have community and purpose, and would be happy to give back to a society that guarantees their wellbeing for rather modest reward, and I really don't think finding enough people to do the actually necessary work would be a big issue at all. Kids that stock up their pocket money by mowing lawns are basically already making that exact deal.
Adding another comment to address this PS.:
Those volunteers are still volunteers inside a capitalistic system that have to get by somehow. Of course they're going to spend their extremely limited free time on the things that benefit them directly (features they need, bugs that affect them). The incentive structure is set up against them. That would be very different if they didn't have the pressure of keeping afloat in spite of their volunteer work.