> That is, it took 25 to 30 years after mass production of automobiles was started by Ford for 50% of "horsepower" to be replaced

I just googled "average horse lifespan", and the answer that came back was, exactly, "25-30 years". There's a clue in that number for you.

Yes, I considered that. Someone using a horse-drawn wagon to deliver goods about town would likely not consider buying a truck until the cart horse needed replacing.

The working life of a horse may be shorter than the realistic lifespan. Searching for "horse depreciation" gives 7 years for a horse under age 12, the prime years for a horse being between 7 and 12 yrs old, depending on what it is used for.

I'm willing to accept the input of someone more knowledgeable about working horses, though!