Soon after I posted this, it was pointed out that a similar question exists. That other question answers most of mine. I shall leave this here as a stub. That way it provides a pointer to the other question and, importantly, my title contains all the keywords 'model', 'quadcopter','helicopter' and 'scale up' whereas the other question's title (Why haven't quadcopters been scaled up yet?) is less comprehensive and therefore less searchable.
Model helicopters were on the market for many, many years before quadcopters.
Presumably most of the engineering bugs had been removed by then as well as control issues.
Suddenly along comes the new kid on the block and it's all change.
Why are model quadcopters so popular in relation to model helicopters?
How come the same reasons don't apply to full-size helicopters and quadcopters? Why don't we see search and rescue quadcopters and personnel-carrying quadcopters?