But I suspect its failure (to keep out international criminal orgs), not economic success, that gets them raided by traditional nation-states.
The Cayman Islands are a British Overseas Territory. A seastead isn't going to be an overseas territory of a significant power in its own right that is also a NATO member and permanent UNSC seat holder.
I think whether that's possible depends mostly on how cheaply these things can be built at scale.
And the fact, that nuking is considered a crime against humanity even when you target libertarians.
Does not mean that it will not be tried, though, as soon as they will be seen as a threat to any big player established. But there are lots of easier options, than plain nuking - "terrorists" for example. "A previous unknown generic terrorist group just has hit Seastate X, how tragic"