People aren't horses. Horses were just a tool, and we found a better one. The economy exists to serve us, not us the economy.

Horses are animals, not tools, but they found an economic role through their labor. Hopefully humans fare better, but as far as the economy is concerned, the difference between horses and people is that horses don't have money to spend; their owners did, and once tractors became available they no longer saw fit to spend that money on horses. The economic role of unemployable humans would depend on how much purchasing power they have access to through means beyond their labor. Economic theory makes no guarantees about that.

Farming used to be most economically valuable work until it was not. OpenAI is not even close to producing anything as valuable as farming equipment.

If we need jobs, we will create them, do not worry about that, see David Graebers work on the topic.