Firstly I want to start off saying that I don't think the rumbling is morally correct.
In front of paradis there four options:
• Rumbling
• Diplomacy
• 50 year plan
• euthanasia
Diplomacy is a non option since for the last hundred years paradis was used as the scapegoat for the eldians outside the walls and Marley basically being eldia 2.0 to the entire world.
The 50 year plan is just delaying the inevitable, say Historia and her children are sacrificed for this and the conduct a partial rumbling, at that point the entire world has a reason to antagonize paradis even more.
So during the 50 years while paradis tries to catch up technologically, the rest of the world would develop more sophisticated and stronger anti titan weapons making the rumbling irrelevant.
There is no way paradis alone can even match the reasorces of the the entire world ao they will never have regular military dominance without the rumbling.
Euthanasia is just killing yourself before they kill you, hardly any option.
So the best option here is the rumbling or specifically the 80% which is the least bloody (from paradis's perspective).