The final English defeat in the Hundred Years War was at the Battle of Castillon in 1453. They were defeated by French artillery. Admittedly, this was in an entrenched camp they attempted to storm, but the technology was the difference.
The basic English tactic throughout the war was to take up a defensible position and then slaughter the French, who were generally stupid enough to march right onto the killing field.
Had the French, for example, surrounded the English at Agincourt, which they could easily have done, and waited for hunger and thirst to do their work, the battle, to the extent it could be called one, would have been a French victory. 24 or 48 hours would have done the trick. Of course, doing so was not possible for the French, as it required a firm command and did not fit into their cultural meme of the glorious charge.
The Little Ice Age and any effects on the strength of the populace didn’t really kick in until the mid-1500s. The longbow was used extensively in the English civil Wars of the Roses, at least their early stages, and was largely responsible for the truly astonishing death rates in some of the battles.
The English longbow was used well into the 16th century. I’m not really sure why the longbow fell out of favour. It was probably the most devastating infantry weapon in existance until the introduction of the mass produced breach-loading rifle in the second half of the 19th century, but once the art had gone, it was almost impossible to get back because it took a lifetime of training and practice to become proficient in the use of the war bow.....