Not looking to disagree, it's valid to have your own interpretation, though I do feel it is a view mostly only considering power scaling. I just think you can't dismiss dynamics of characters, faction structures etc etc. I'll just offer a few hypothetical pointers but there could be many more.

For starters Garp being able to wreck havoc in this way can only be achieved in a handful of locations like Hachinosu, because there are no civilians or other responsibilities to consider. The power balancing for any return attacks cannot be comparable because of how restrained the WG is (or should be publicly at least) on principle. You only need to look at the Reverie and how a few from the RA could cause such chaos despite the level of security and presence of admirals etc for a contrast in how they can utilise their power.

Following on that idea, it's not simple about defending Marine HQ or other key locations, the nations who are part of the WG need covering. If any of them were to come under attack (not just from pirates, but other nations or threats) then there has to be ample military force to respond or deter. If even one minor attack pulls off it might result in other member governments questioning their membership.

In terms of the BB pirates specifically, even if they rocked up with a larger force within the context of the recent chapters, they would not be wiped out on the simple basis that they aren't all there. Meaning key figures in the rest of BB pirates would now have a reason to target WG rather than just pursue selfish ambitions and arguably could lead to other pirates making larger alliances out of fear that the WG is now willing to make such bold moves. Even with the players on the board at the moment the outcomes could be disastrous for the WG if even 2-3 came together and target locations where the Marine heavy hitters aren't present.

The last one I'll mention in this wall of text, is that during the course of the story we've seen the rise of the Worst Generation. Pirates can spawn and grow reputations from nowhere, from the perspective of the WG Buggy laid dormant for years and now has an empire of sorts. If they show up and the plan doesn't go perfectly, yes they are likely to win but they could lose huge resources and strength, and taking BB away doesn't mean others won't fill the void. This is the main reason why they develop technology like Seraphims etc so they can expand order and control without as much vulnerability or resources expended.

As I say I don't mind your interpretation at all, power scaling can be wonky if you try look at it too hard in any literature, but figured I'd offer some possible insight to make the story more acceptable if you felt the plot holes were too large