The reply to this question rambles on to the meaning of “gone south”, but never really gives an alternative to the idea that would conflict with the migration of birds, except the idea of fleeing to Mexico to avoid the law, which IMHO is not very plausible.
A map, properly oriented, shows north at the top and south at the bottom. A profit and loss chart shows the positive numbers at the top and the negative numbers at the bottom. A loss is shown by a move in the “southerly” direction. Profits have “gone south.”
For my money, Evan just plain fluffed his answer, in The Word Detective. Perhaps he was trying too hard to stick with gone South rather than to go South or one of its variants.
Mathews cites sold South from Uncle Toms’ Cabin in 1852. He also cites a journal from 1902, which was referring to 1746 which said in effect that Indians spirits would “go southward” when they died. Supporting this last cite, there is an 1894 cite which says
It does, doesn’t it? Unfortunately, Cecil would rip me a new anal aperture for it because I made it up. No cites. No research. It just sounded reasonable and obvious.
Gee, I hope it doesn’t end up in one of his books!