joel moskowitz
I don't disagree with most of what you say.

This I can actually answer:

"And the clear question that still comes to me is that double iron planes were not "cheap and common" when they took over, and they weren't even cheaper than a steeper pitch plane once they were, but most craftsmen still seem to have chosen them, anyway. They were more expensive. Why then was everyone willing to pay more for them in a time when craftsmen had very little disposable income. "

for the same reason - power sanders were more expensive than planes.

Less skill. less cost. For the average workman - doing joiner's work, the plane will last a lot longer as the mouth opens with wear but the plane still works pretty darn well for the type of work it is supposed to do. There is no evidence to suggest one way or another that on the super high end of work double iron planes replaced single iron planes before the 1820's. Not enough people did super high end work and there is not enough evidence.

Andre Roubo in the 1780's BTW asks a different question. He wants to know why iron mitre planes were not generally used in general cabinetry instead of marquetry. Of course he was just addressing himself to high end cabinetmakers not joiners. 10 years later English makers started making iron mitre planes in more common quantities so they began to be used for high end cabinetry.

Duncan Phyfe had an iron mitre plane in his toolbox from the 1820's although how important it was in the actual workshop isn't known.