you assume that the only reason - or, even, the most important reason - that people work is to make money.

No, I do not. You're wrong.

I assume that the work that people do for money is important for the society and that a lot of it wouldn't get done if people worked just for pleasure. Basically, without money you'd get too many DJs and too few plumbers. Money fixes that balance problem.

Believe it or not, many people farm or tend gardens or animals simply because they enjoy doing so.

Of course, so what? Small-scale agriculture is remarkably inefficient. Specifically, it cannot feed the current population.

that humans will always choose meditation or video games or somesuch over 'mucking in the dirt' if given the choice.

Not always. But too few people will choose mucking in the dirt and without money I'm not sure how are you going to persuade a sufficient number of people to go and do what they don't like.

Experience shows that systems like these do not run into problems of food shortages (in fact quite the contrary).

Do tell me about that experience. I'm curious.