Old-time native Southerners frequently attribute the ruin of the south to air conditioning. Undoubtedly air conditioning had a huge effect on the influx of population to the southern states, but I think another event - not an invention - is the main reason or at least a major contributing reason.
Ellis Arnall was elected Governor of Georgia at thirty-five years old, winning him the title of "Boy Governor" He was what would have been called a "Progressive" or "Reformer" or some such other title that bespoke his unorthodox views. He pushed through the law making Georgia the first state to allow 18 year-olds the vote and made the operation of the prisons independent of the Governor.
The thing he did that probably had the greatest effect on the whole country was to win a lawsuit against the railroads. Before Arnall's victory against the railroads, the south was maintained as an agricultural colony of the north by means of discriminatory freight rates. If a planter wanted to ship peanut butter to the north, it cost him more than if a northern company shipped the same peanut butter to the south. If the planter wanted to ship unprocessed peanuts to the north, it was cheaper, thereby protecting northern manufacturing - and unions - by means of higher freight rates on finished goods.
If a manufacturer wanted to move to the south to escape union control of his company, it would make him uncompetitive with his northern competition unless he could manufacture his product considerably cheaper in the south.
The railroads claimed that they had higher costs of road-bed maintenance in the south than in the north and other economic reasons for the higher rates from south to north traffic, but Arnall showed that that wasn't true.
Some other southern states refused to join the suit because they feared that it would be unsuccessful and the railroads would retaliate. As the saying goes, "No guts, no glory."
I have not done a study of this (and don't intend to), but I wonder how much of the Rust Belt would still be churning out manufactured goods if Arnall had not broken the freight rate disparity. Soon after Arnall's victory many companies started fleeing the north and union control.
Demographic shifts probably always have many unseen causes - people are still debating what caused the fall of the Roman Empire - , but when companies can move from a higher-cost to a lower-cost area, they will probably do so, all other factors being equal or more favorable.
The next time somebody is complaining about how the south is going to hell in a hand basket, they can lay at least part of the blame on Ellis Arnall.