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We are not in a static equilibrium. I don't think anyone believes we have reached an ideal state of technology where we don't need to progress any further. People who are not producing things that are immediately useful can be working to improve processes for building things, inventing new things, doing basic research, and supporting people who do that.

There's also the issue that the population is constantly growing and in flux in different areas. Nobody knows the exact right amount of stuff to make to serve everybody and underestimating can have dire consequences and get lots of people killed, so we can and should make more than might be necessary.

It's a huge oversimplification to say we just need 100 units of food so get the 5 people to make 20 units of food each and then we're good, no need for anyone else to work.