Despite the attempt to simplify labor to the point of fiction, I can roll with this example to explain why I see labor vastly different from you. I have a lot of questions about this set up. How crucial is it to move these rocks? What's the deadline? How many rocks need to be moved? Are there safety procedures in check, and will safety equipment be provided?

You as an employer want to move as many rocks as possible as fast as possible with as few people as possible. The people comprising your labor force want to move the correct number of rocks on a reasonable timescale with adequate resources and be recognized as valuable individuals. If your needs from the foundational questions are out of line with the worker's needs, then you are not guaranteed that labor, and you don't deserve that labor. The factors that determine if the labor is essential are the workers and society, not the employer. If the workers or society don't deem the labor to be as essential or as worthwhile as you think, then you face delays or other factors. In a cooperative or otherwise non-market economy on a larger scale, this would appear as degrowth, which is normal and ok. Not everything is essential, and even in periods of degrowth there's still plenty of labor that can be done. All labor is specialized as well, which is an additional problem I had with the initial objections. People are flexible and want to provide meaningful labor. If there's a period of degrowth and the task of moving rocks becomes a completely unnecessary for the current economic needs, the people providing labor will seek out new meaningful labor. The needs of labor always change, but labor itself is a fundamental and unchanging aspect built into our natural instincts for survival.