Well that's my issue with this narrative.
Why WG wants to sink whole world? Cleansing doesn't mean sinking whole world when WG wants to rule it.
And, if they did cleansing in the past and weapon does cause sea level rise then WG should be aware of how much to use it to avoid whole world sinking.
So, WG deliberately causing whole world sink doesn't make sense to me
“Think of human lives as nothing more than insects. No matter how far their numbers dwindle, it's always possible for them to multiply again.”
- Saturn
A bunch of kingdoms are in active revolt, Mariejois is running out of food because of crippled supply lines, etc.
I think the CDs are more than willing to wipe up 99% of common folk on a whim. The balance of power is just a construct to keep them fat and comfy (pirates attack common folk -> common folk sign up for protection -> common folk give resources and taxes to government -> government allows pirates to exist -> repeat).
The moment they’re even slightly uncomfortable they can just wipe the slate clean.