Breeding makes more and more sense the longer one breeds with commitment to a specific variety of a specific breed with specific criteria.
There are some influential, internet poultry folks that like to spin yarns, and that's cool; it fuels the imagination. Nevertheless, at times the yarns spin off into an alternative reality.
"Dual-purpose" is not a specific measure; it is a spectrum with a lot of variation.
There are many, many different records, from university experiment stations and governmental programs, that establish a fairly clear historical understanding of poultry development over time. Knowledge of breeding poultry for production catapulted forward starting in the early/mid 20th century. Before that there was very little of improving anything that wasn't sheer luck and the accumulation of anecdotal understanding, some of which was spot on, and some of which was completely incorrect--sometime efficacious in spite of ignorance.
Reading fantasy into history is rather fun, the largest market in current literature proves that; however, if our goal is to understand poultry evolution, a lot of that history needs to be taken with a grain of salt; e.g. la poule de Bresse is comes from the region south of where I used to live in France. They are very proud of their chicken, but that phenomenon is rather recent historically. La Fleche and Houdans are much important historically, although it is true that the Bresse has surpassed them in importance currently. It's not because of anything historically interesting with the Bresse fowl; it's rather an expression of modern poultry science being applied well in a region with farmers that chose to do so. They're cool, but their certainly no more chique than a NH, and I'd argue much less beautiful and much less refined.
Excellent producers, in a current sense, are specialized producers; the produce meat or they produce eggs to an excellent degree. They don't produce both, and in the time of the true "dual-purpose" fowl expected less of their fowl that we do today when we say production.