allocating a few days a month to all fit members of a community to do work which no one volunteers to do.

Ok, this basically sums up the answer: the community forces labor one way or another. What is the enforcement, carrot vs. stick for making people do their fair share. How do you reward people for doing unwanted work? How do you deal with someone refusing to do it, or “maliciously complying” and doing it terribly to make the job easier and/or get out of doing it again in the future?

So the agreement is that there is work that needs some external impetus to happen, because not every job has enough people intrinsically interested or civic minded to make it happen. The question becomes which solutions manage to be more fair than others? For unskilled and unwanted jobs, the current answer has a lower class overworked because they are the most desperate, and that’s bad. A forced labor system might manage to distribute the burden more fairly, though thanks to people being crap it’s likely for a system set up to do that to be abused to overwork some demonized demographic, ending in a similar outcome a different way.

Whatever the case is, it’s not as rosy as “people freely work on wikipedia and programming, therefore people will freely work on anything society may want or need”