That the return (assuming no sale at top of a bubble) for the best companies in the biggest industry of 19C was only 5%-10%. (Based on equity, it seems Harford did not calculate dividend return which may make a difference)
Making 15-20% without an exit is a great deal - supporting I think your point.
Indeed, I agree. I'd be curious to know how common was the use of debt during that period. 5%-10% is a good return on an unlevered asset, debt could provide an additional turn - then, I don't know how the Kd and CPI was during that period.
During the 19th Century, U.S. railroads relied primarily on debt issues to finance their growth. This policy contributed to major financial crises (www.biu.ac.il/soc/ec/wp/16-01/16-01.pdf)
Making 15-20% without an exit is a great deal - supporting I think your point.