That coal hoppers went off line is not in dispute. That they did so "in
large numbers" is relative. The fact remains that most photos of coal
trains in coal country show very high percentages of home road cars. OTOH,
research has demonstrated that coal moved via certain routes off-line, e.g.
to the Great Lakes for export. But please note that such is NOT the same as
"going everywere off line," as we might expect with XM, FM, etc. cars.
I think if you want to model off-line coal hoppers, you need
documentation of what you choose to model. As someone else has said, the
favorite modeler's coal train in which every hopper is a different road is
plain silly. The farther we get from that, the better.
(All spoken, of course, by someone who has close to zero need to model
ANY coal cars of any description.)

Tony Thompson Editor, Signature Press, Berkeley, CA
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Publishers of books on railroads and on Western history