9-12 inches in 80 years!
Wow how can we ever cope with that?
Coastal cities will still be there. Most no one will notice.
Every time people feel the need to speak and remove all doubt about their powers of understanding the way you do, I point them in the direction of insurance premiums in the affected areas.
We are at a point where opening a business in some coastal locations might not be viable because insurance is simply too expensive - if you manage to get it at all.
Educate yourself.