In fact in the real world existed many technologies, that got lost over the centuries, and cannot even with current level of technology and science. There are of sort of such things: from legendary Greek Fire and impressive architecture to ways of food preservation and extremely precise flint knapping. So there really is nothing strange in fact that no one can recreate certain technologies or devices from ancient time, despite higher level of science and technology and no genius like Vegapunk can help.
As for question why somebody build something capable of destroying whole world… um,... hey we did too, during last World War. People don't necessary know consequences of their newly developed inventions would or could bring, and even if, their sometimes are desperate enough or sick enough to take their chances.
Besides "power able to destroy the world" might not mean the power to destroy the planet, but power capable of destroying world social and political order, as "the world" people live in is the world of relations between them. At this point we really can't tell which route Oda-sama will take.
,could be both type of threats?But nah, just the name 'ancient weapon' implies physical destruction already so when it comes to the weapons at least I believe the world destruction threat is literally that)
