Sm-6 is getting a bigger, fatter sm-6 IB (on 21" booster) precisely with larger warhead in mind...but 64kg isn't a joke either.
Many PLAN weapon choices(as for any navy) come from its history. Sometimes it's as if PLAN always was that it now is, a huge force growing to rival USN. It wasn't visible 10 years ago, and even 5 years ago still wasn't this way simply because changing yourself is longer than building a ship.
PLAN simply was a defensive "salvo" force just a few years ago - and still maintains this direction of development as a plan B - it isn't anywhere near parity with USN§co yet (safe check+some institutional inertia+making use of many perfectly fine hulls).
Plus, if you already developed something - using it isn't bad. Dedicated common shells were better than AA ones in anti-surface role during WW2 even. It was simply better to carry all the possible AA shells, and use them for a surface contact if it presents itself.
By having more large SAMs onboard - you lose some anti-surface capability(not the ability to prosecute the engagement, only lethality and ambiguity of launch point), but pay for it with at least 1 full AA engagement.
It's quite a tough thing to ask.
The more PLAN composition develops towards being a 1st class force, the more it'll make sense to offload offensive capability towards aircraft and nuclear submarines.
It doesn't mean destroyers are bound to lose it, but their importance for ASuW will be going down.
