So, the argument is that you can eat if the previous three generations scraped enough away for you to afford food? ... Cheap food is still expensive if you have no money and no job.
No, I'm assuming that even in a society technologically advanced enough that almost everything is done by machines, there will still be ways you can make yourself useful enough to someone to earn what you need to survive. If nothing else, you can always grow your own food, build your own shelter—it's not much, but with knowledge of modern science and access to cultivated seedstock you'd still have an advantage over most humans throughout history. Of course, we're talking about the bottom 0.01% here, under the assumption that they don't have any marketable skills whatsoever and can't rely on charity or salvage to get by, none of which seems very realistic to me.
As you've already pointed out, a commune cannot exist if people can choose to join it or not. That's the whole point of communism. If you are fairly skilled/etc then there is no incentive to join, and thus the commune fails since only those who cannot provide for themselves join.
It goes further than that. The commune has to do more than just force you to nominally join; it has to force you to contribute. Without, needless to say, offering positive incentives conditioned on your contribution, since that would go against the whole point of a commune. The commune essentially has to consider each individual's skills and labor property of the commune rather than the individual, with failure to contribute according to one's ability punishable as a form of theft from the commune. As I see it, a system where each individual is a slave to the group is no better than one with distinct slaves and masters.
Communism can really only exist if it is imposed at a societal level. That's why communist societies tend to be associated with atrocities - it takes a very authoritarian government to sustain communism for any period of time.
I'm glad to see that we're in agreement, then. I thought you were advocating a communist society.
Why would anybody who is productive agree to share part of their income with the rest of the co-op? They have no incentive to return the favor should the tables turn.
It wouldn't be up to them. The organization's charter would dictate its purpose as providing a basic income to as many people as it can afford, starting with those most in need, after targeting a particular rate of growth. A bit like a trust, really. The idea would be that you start with a some donated seed money, say $10M. That gets invested at a real return of, say, 7%. You want to target 6% growth, so that leaves 1% of $10M ($100k), which you split five ways to provide a basic income of $20k each for five individual members. Assuming conditions remain unchanged, after 25 years you could support up to 21 members. After a century, nearly 1700 members. After three centuries, 3.5 billion members. Of course, the real world won't be this tidy, and there is plenty of room for fine-tuning, but the basic principle seems sound.