IMO that's not a valid argument. For instance, 807 cannot
survive without DDs, 806 needs the DDs for PM more than AM. 800/811
has very few DDs (1 or 2 on most days), 803/805/812 have none, 804
have a few.
AMDEP needs to take in more DDs and kick out more rigids from
its fleet.
800 804 806 had been using NWAB old bendies all those while,
similar to Woodlands feeders as SMRT has a stand of not deploying
DD on feeders for now.
It is the upcoming Seletar package that Yishun feeders
benefitted using new DDs while Woodlands side continue to use those
old bendies. Ended up the old bendies from 800 804 806 are running
around other services now (including 857). 811 812 still using
bendies though.