Wording of OP title is unclear economically speaking
The only party involved that can actually affect SkyMiles "demand" is Amex and other transferrable credit card companies (if there are any). They are the major players that purchase and therefore "demand" skymiles. The only way to prove that Skymile demand has increased would be to demonstrate that Delta's sales of skymiles to Amex has increased. This is probably indirectly true when viewing OP's last bullet point with Amex spending up double digits (hence increasing overall sales of skymiles from delta to Amex).
Flyers using more skymiles has nothing to do with skymile demand.