Probably unrelated to AMEX AU changes. I think devaluation for AMEX is just because SQ is selling miles at higher price to AMEX/inflation/AUD exchange rate lowered against SGD etc etc.Slightly OT, but still related as it's an Asia based FF devaluing:
CX Devalues AM from Oct 2023
So maybe CX saw AmEx AU doing this and joined the bandwagon, or have their own rising costs leading to this... It's anyone's guess.
But definitely CX wants to clean up the balance sheet, and increase margins. Capacity still tight around China and surrounds so opportunity cost for redemption especially in premium cabins definitely has gone up. A lot has changed since 2018, so this is just a normal inflationary adjustment + squeezing out a few extra dollars post-covid.