In one of his travel books, which I read at quite a young and impressionable age, Mark Twain relates how Burns's mother, upon being shown a large memorial statue to her late son, shook her head and said "Och, Robbie, ye asked them for bread and they hae ga'en ye a stone." From that, I always assumed that Burns died from hunger and want after a long life of penury and I'm pretty sure that in the 19th century that formed part of his mystique. The extreme poverty notion seems to have gone away now, I hope correctly.