I think the main competition for Y+ is not CX Y or CX J, as the fare gaps are quite large between those cabin class. Y+ competes with other airlines that offer 1-stop Business Class. So passengers can choose between whether I want to fly Business Class with a connection, or nonstop with Premium Economy.

For example,

HKG-JFK
CX Y+ HKD 28,000
MU J HKD 18,000

HKG-LHR
CX Y+ HKD 18,000
SU J HKD 25,000

HKG-SYD
CX Y+ HKD 18,000
MH J HKD 22,500