Then why haven't the Emperors destroyed the WG if they are so mighty? Because they aren't. They are pirates. Criminals, that is. The only reason why Whitebeard could pull off the attack was Ace. And even then he got backstabbed by one of his own men. The threat of a horrible retaliation indeed would be very real, but by no means assured. Even if an Emperor in name commands lots of men, ships, and power, it's not like the true military of WG, it's just a bunch of pirates whose own pockets will always be closer than the common good - unless you really get them thrilled like by saving Ace (and are a fricking Whitebeard who could turn a regular forest into an army of ents to follow him by sheer inspiration).

But still, this has got absolutely nothing to do with what I was saying originally: That the Fishman island goverment seems weak and the rebels are at least partially justified in wanting a strong one. Even pirates have three strong "nations", so it's a disgrace the fishmen and mermen don't.