I've seen some people have issue with why the government/Im would keep such a powerful weapon secret or what the downsides to this must be. Personally, I'm already good with what we see here and basic assumptions. Using such a powerful weapon publicly and regularly would drastically change the world order into something completely different and filled with fear and stagnation. Obviously if Im wasn't using it regularly, that means that type of world isn't what they want. They want the world as it already has been, thus this weapon is absolutely reserved for only the most drastic situations (to hide their own identity in this case) and in ways that can still keep it a secret (so the world can actually function outside of being terrified of this threat). But that's just my personal rationalization, will have to wait and see if Oda elaborates.
Something I considered when thinking about this is that the WG needs there to be people, so they can't just use this weapon to obliterate everywhere that opposition forms. The Celestial Dragons are pampered brats that don't have any life skills of their own. They rely on slaves and money to get what they want or need. Less civilians means less people who can cook, build, pay tribute to the CDs, etc. Similar to the concern Jack had about Kaido drunkenly destroying Kuri.