They shouldn't even have the ability to opt out. Only way to push through real change is to really be harsh on these people - you don't get a dime unless you accept the government compensation for your land (which will be rock bottom because these lands are unfit for long term human habitation) and move. Some of these parishes in Louisiana for example, there should be no new development. It's hard to push that sort of thing through without a taking requiring excessive compensation, but again the government is not very creative. Stop building schools there, stop building roads there, and pull funding from anyone that does. Most people will clear out quite quickly when they realise they can't get their social security check mailed to a parish that's underwater every other year.
Yes America has a deep problem in that rural people have an absurdly outsized political influence. It's the same reason massive cities with millions of people don't have basic infrastructure, rural shitheads refuse to vote for the funding. We are a modern urban economy run by rural peasants who extract massive rents from the actual productive and populated regions of this country. We have to stop bailing them out at some point. I just don't think cutting insurance will do that much. There are a large number of people who would happily move if they had the money or means to do so, so the government should start there before getting into court battles with rich people and rednecks over the value of their f**k swampland.