The answer is not static, because technology is not static. Automation continues to improve, and even faster change is coming. Culture meat, cellular agriculture, controlled-environment agriculture, etc. So you'll need ever-fewer people working in agriculture, far fewer than work there now (globally), but not zero.

But construction, manufacturing, even sanitation are much harder to automate. We're a long way from a robot being able to clean up a messy house. Think of what it would take to deal with this properly. Cleaning out the garage, sorting out the garbage from what had value, maybe listing the reasonably valuable items (if any) for sale, identifying what may have sentimental or novelty value, etc. There are tons of jobs like this out there that are labor-intensive but not easily automatable.

I also don't see humans "evaporating" suddenly, unless you believe in the Rapture. I don't think the rich are plotting to murder us all. There are, however, plenty on r/Futurology who aspire to kill hundreds of millions of people, or engage in radical population reduction, to "save the environment" or achieve economic justice. I just don't think effectuating that is very likely. I think a more likely issue is population decline from lower fertility rates. But that decline will be relatively gradual.

https://ourworldindata.org/fertility-rate#what-explains-the-change-in-the-number-of-children-women-have

I do think that may eventually lead to the collapse of technological civilization, but I also don't see any remedy for it. Mainly since most of the things demographers trace the declining fertility rates to I consider positive developments. Education, wealth, empowerment for women/girls, etc.