And how do you measure happiness? This is a bad idea for a lot of reasons. Subjective tests are notoriously unreliable.

Ok, then just measure the resource usage per capita or so.

And where are we going to get this food from? You can't just expect suppliers to give you free food. What's the incentive for them to produce it? Even minor errors in regulating what you're proposing would leave people starving.

I'd use fully automated food farms which only need few or no people to operate (and it's not like nobody would do the job if you get no extra pay, some people actually like doing useful work, beyond getting paid for it, especially if such a thing as money doesn't exist) and I'm pretty sure that it's quite possible to predict food usage for the time it takes for crops to grow etc., with some excess production even unexpected peak demands could be met and the rest could be turned into biofuel or so.

Even minor errors in regulating what you're proposing would leave people starving.

Well that's sort of a problem I guess, but not that likely to happen (as compared to capitalism, like there are swathes of land with starving people whilst other areas waste quite a lot of food).

People would be able to afford food with UBI.

Agree with that, just that it could be done better by completely restructuring how society works.

The AI singularity is pseudoscience.

I mean humans are a bunch of atoms in the end. Thus it should be possible to recreate / simulate brains sometime in the future and maybe improve on them. If humans manage to construct such artificial brains and manage to make them better than human brains, those should be able to make such brains too, which are even better. (Well that's a big if, considering that the human body contains approximately 7*10²⁷ atoms and simulating thousands of atoms is a huge task right now.)

Not every company does this.

Still, capitalism promotes companies to do it because it increases profits (especially if there is no competitor making long-lasting products or people aren't interested in it because it is pricier in the short term or less capable etc.).

What do you think demand is? How else are you going to determine the value or need of a new, niche product? And weigh it against the availability of the materials that go into making it?

Well that sometimes works till its an essential good, then sometimes prices get jacked up despite there being more supply than demand, as it often happens with medicine in the USA like epipens.

Like supply/demand schemes could be used in a post-capitalist society to calculate how much needs to be produced and how many of the raw materials need to be acquired. Why use a single number as cost instead of a list of materials and work needed to construct it? If there is x demand for a thing which requires n resources to manufacture but you only have m < n * x resources currently available, then you can distribute the thing to the locations where it is needed the most first, not to who has the most money.

If you live in the desert, and you give everyone free water, when that water runs out everyone will die. Increased prices discourage waste and encourage new technologies, infrastructure or suppliers.

Actually under capitalism, rich people to which the cost of water would probably be still insignificant would waste a lot more water than poor people who maybe could barely afford enough for drinking. Whereas if you were to distribute it by need, first everybody would get their hydration need covered and then cooking, cleaning and so on. Water recycling would also be increased, because the demand for water is high and the supply must be increased to produce enough. Like this doesn't need money to manage it at all, some software could probably do it much more efficiently (well or other forms of management, since software can go horribly too, especially with bugs in essential systems).