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.