Your argument is that the airlines had a prescient view that their revenues would decline relative to their competitors, and before that happened implemented baggage fees to partially offset that fall.

With natural experiments like this it's painful to disentangle cause and effect, but with the timing (fees, followed by revenue declines) I'm inclined to see the author's explanation of fees being a more likely cause.

All large businesses try to predict their future revenue. An airline that predicts a future drop in revenue is more likely to add baggage fees to boost revenue.