Well, this has been an education. Frankly, there's more information here than I ever wanted to know about plumbing systems, but I am impressed.

John, I don't think your statement that "Radiators" are still found in a few older homes, but most of them get converted to "forced air" as the old systems break down holds true in Minnesota. In my neighborhood at least, old-fashioned radiators are still found in most older homes. (In my neighborhood, most homes are older.) In a few older homes, radiators have been replaced by baseboard heating systems which are essentially radiators of a more modern design. Baseboard heating can be installed while leaving the furnace, boiler, and most of the original pipes intact. It would be a much more drastic and expensive remodeling job to replace radiators with forced-air heating, because you'd have to rip your walls apart to install the air ducts, as well as replace the furnace and boiler. I don't know anyone who has done that.

Newer homes tend to be built with forced-air heating, though.