I agree. It was originally a throwaway reference around how it factors into peoples' motivations to work (in my opinion), so I'll cover this later on.
So I'm going to move away from the incentives (and discuss that in the last part of my post) and towards the journey. First off, yes, it sounds like a pipe dream, because of the difference between the state we're theorycrafting about, and the state(s) any of us live under now. That's why it sounds like a pipe dream. That's the underlying tension in how believable it may or may not be, even to folks actively considering the possibilities. I get that.
However, the journey is what makes it possible. At the moment the tension between automation and jobs is that people need jobs to live, and automation can be used in place of peoples' jobs, making them (at times) unable to pay for the basics. This would not be an issue in said theoretical society. People would be catered for. Therefore, we could invest more in automation.
Is automation going to fix every problem? Not at all. I'm happy to theorise on a case-by-case basis, but it would fix a lot of problems. A lot of administrative work, when it's not created on purpose, can absolutely be automated. We already have the technology to create machine-readable forms. We already trust electronic signatures (in specific contexts where the contract between the people involved is a known quantity - I'm not going near something like "voting" for my argument here, just like "yes i have digitally signed this receipt from my secured email" kind of thing). These are things we can work at automating, or flat-out eliminating in the case of unnecessary bureaucracy. Cleaning, likewise.
So much innovation is stifled under capitalism because it's not profitable. Or because it threatens someone's existing bottom line. Call me an idealist, but with that removed, and combined with our ability to innovate (look at what we've achieved as a species within a century - a lot of which was done for the theoretical good, and some obviously done under more compromising circumstances, like for money or for a war effort).
Cleaning, again, there have been arguments to this throughout the thread. Folks want their places to be clean. The issue is public spaces, but even under capitalism I can tell you, nobody pays to fix public toilets up properly. At least here in the shining beacon of modernity that is the United Kingdom

I'm happy to chalk it up to "why don't we try and see", because it's not exactly something that's solved
now that would then not be solved under this new socialist-or-communist-society.
Back to incentives, then. Toilets need to be cleaned. People (let's say) don't want to spend their time cleaning toilets, nobody wants to share a rota, etc. Let's assume any human-centric resolution this problem could theoretically have isn't going to work out.
We're
really good at working out solutions for stuff we don't want to do. And with more free time, more minds are able to work out solutions to problems. That's why the matter of "incentive" needs a different framing to the society we currently live under. This stuff happens in stages, we don't immediately just somehow create the end state. I am confident that the species that inside of two centuries worked out computers, powered flight, space travel, splitting the atom, etc . . . can figure out a reasonable solution for sanitation. Because it's a
shared problem. Everyone would feel the impact of poor sanitation. Redirecting even a fraction of humankind's innovative output would definitely come up with ideas. The lack of capitalism would also mean it wouldn't need to be profitable to be possible in the first place.
I'm not an expert in it. I can't tell you what that solution will look like. I can speculate at quite some length at what software would look like independent of capitalism, because that's my wheelhouse. Individuals cannot be reasonably, fairly, expected, to be able to model every single aspect of a theoretical system. But people have still, in this thread, including myself, at least
tried to present plausible solutions, or paths to get to a point where a solution is possible.
The whole thing about "incentive" misses the point about what would and wouldn't be necessary in this hypothetical future. At the centre of it, your disagreement is with the amount of hours that are needed to "keep society going". You think it's a number that can't be reached without sufficient incentive, and only our current system, or some modified version, can provide that. Or, at the very least, communism (or socialism, even) cannot. That's probably more accurate. I think we're back to "nature" again, I don't know. Do you believe that humans
need both a carrot and a stick to get things done? Or do you believe something different?
In my opinion, the people that are motivated to do the things that matter to them, and / or that they happen to specialise in, are all the incentive we need. I think we're an inherently industrious species. That's my view on incentive, generally speaking. Very high level.
I don't think we should worry about the ones who don't want to work. I'd be more concerned about those looking to sabotage the work of others, which we see in the here and now. The "bastardry" that keeps being mentioned. But again I don't think it's some unsolvable problem. It doesn't defeat the concept. I deal with people who are more concerned about how they look, than the actual work they do, all the time. They don't bring down the company I work at under capitalism (and indeed are often rewarded for presenting a successful image), so I don't think they'll bring down people working together to build whatever thing is being built, in the hypothetical future I'd like to see (in some form).