> demand for horse work dropped rapidly and never recovered.

I don't think horse work is a problem. If anything, now they can enjoy their day wondering large vast fields in the summer , and eating high quality alfalfa hay made by a tractor in the winter. And since the invention of the Horse Trailer, they can travel with their owner on interstate highways at speeds as much as 55mph while eating the same high quality hay going to another field.

If anything, the horses benefited the most from the technology improvement.