If someone else is going to fulfill my "needs and wants" (that latter word is the really key part) without demanding anything in return, I'll spend the rest of my life engaging in activity that is productive and fulfilling for me, but not necessarily of value to anyone else (i.e, I won't be fulfilling anyone else's needs and wants, just my own). Certainly, I could choose to produce things of value to others for purposes of ego gratification and praise, but that's a weak motivation, at best, and it presumes that the things I *want* to produce are of sufficient quality that people will "pay" me for them in the form of praise and respect. This is not guaranteed; the things I'm skilled at (to the point where people would rather have me do them, than do them themselves) and the things I'd prefer to do if I didn't care if I got paid or not are not always the same.
If I can't get my "needs and wants" fulfilled without offering something to someone else in return for their time, labor, or knowledge, then it's still capitalism by another name. If I am told that I will be given 2000 calories a day in the form of processed protein paste, a cot to sleep on, and the most basic of clothing and medical care, thus fulfilling my "needs", but anything beyond that (my "wants") requires me to produce something of value to others, same thing. The more generously you grant my "needs" (better and more varied food, access to entertainment, private living quarters, the resources to pursue hobbies), the more you dissuade me from working for anyone else's benefit. You might argue that with advanced technologies, only a very small minority will be needed to maintain the system, and there will always be those who will choose to be in that role, who will gain enough pleasure from the notion of "service" that this is what they will choose to do so without being compelled. This is almost certainly true. The problem, of course, is that such a structure also attracts those who, by nature, gain pleasure from others serving THEM, and they will then use their elite position as maintainers of utopia for their own benefit.
In any event, it's a foolish notion, because there is no such thing as a post-scarcity society. Scarcity is an ever-moving target. There will always be things where the demand exceeds the supply, whether it's a house with a particular view or in an especially nice location, seating at a live event of any kind, original works of art, or the personal services of skilled professionals whose time and willingness to work is less than the desire of others for their work. The instant you have any kind of scarcity, you need a means of exchange, and the nature of humanity is such that no matter what system of exchange you create, some people will be better at manipulating it than others, and some people will find their greatest skill is the provision of the service of "manipulating the means of exchange" -- that is, middlemen. No amount of declarations that all work is equal will alter the fact that some things will always be more valuable to one person than to another. Thus inequality is inevitable, no matter how egalitarian the starting point. Attempting to prevent the development of markets, however unofficial or called by whatever name, by law or regulation, simply hastens the process, as those most able to manipulate the system become the lawmakers and regulators.