Well done on your carrots, and I wonder the same thing abut managing without modern aids.
However…. there are other points to consider. For example I have not meshed carrots at all this year and have experienced no fly – they are having a year off in Somerset.
Brassicas: we don’t know how prevalent the butterflies were in olden times, perhaps with less rape grown, they had less hosts and were less prevalent. Also with cleaner air and soils, and home-saved seeds, perhaps plants were stronger. Perhaps people ate less than we do! On the other hand there were famines, so maybe not.
I wish I could see a garden of three hundred years ago just to have an idea of plants’ strength and size. And how much pest damage there was.
Perhaps before the days of fossil-fuel-burning, radiation and other pollutants, plants just grew more strongly. But I don’t know how to compare.