Maybe it's not so much the "decline of SQ" as it is the "rise of the other airlines?"
Suites (hard product) have been one-upped by EY
J seat is no longer industry leading since many other airlines already have 1-2-1 (I tried AF's new J the other day and I prefer it to SQ cos of the 'reverse herringbone' layout)
Ground experience inferior to AF @ CDG and a host of other lounges / airports.