why did the wg want to get pluton blueprints so bad if they already had one. this just doesnt make any sense
It was Spandam who wanted them so badly, Gorosei said No initially,
And He was Planning to Betray them once He gets it,
There Existed Poseidon before, yet We got a New One born now,
So just because Pluton was built in the Past, doesn't mean We won't have someone born now who Inherited Intellect of Pluton's OG Creator,
Didn't Vegapunk said He was born Smart? And this Advantage led to what? Creating Mother Flame,
(Shirahoshi for example didn't Create Sea Kings, She is simply who can Weaponize them, same as Vegapunk with Pluton)
Also, as you just mentioned, Pluton Blueprints Exist, so the World wasn't Denied of a way to Build such Battleships, the Problem was in who Fuels them? Similar to how Sea Kings always Existed, meaning the Threat itself never disappeared, but the Problem was in who can Lead this Army?
We recently learned too of those Giant Builders, who are Theorized to be the ones capable of building such Massive Ships, and where are They now? Frozen in a WG Facility (Punk Hazard), so it makes sense that They were forced to build some Battleships (Maybe 16) similar to Franky's Fleet of Battle Frankies from his Flashback, and then They were Frozen just in case, while WG or whoever Kidnapped those Giants just waited for someone who can Fuel them properly (Vegapunk)
The point is, The Weapons themselves never ceased to Exist nor how to make them (Sea Kings are still being born, and Giant Shipwrights still Exist & probably same for Uranus), but what They Lack is who can Use them (Shirahoshi + Vegapunk + ?),