If we create a new island, what about that new coastline? Is it worth preserving for the sake of it's "rarity"?
Development of this land hasn't been off-limits historically, although many of these places in the U.S. are now (rightfully) preserved. Certain types of development are more detrimental to ecosystems than other types.
If something is rare and good, it should probably be preserved, regardless of how natural it is. In this case, we're talking about natural landscapes and living beings that cannot be re-created once they are gone.
Using that definition, anything and everything could be justified as rare.
Coastlines are the most dynamic places on Earth. They change, they adapt, they are a constantly shifting equilibrium. Geologically, they are the youngest and most easily replaceable features on Earth. They're not fragile things.
What isn't natural is how static we've tried to keep them.
>If something is rare and good
You're romanticizing things. Coastlines serve an important ecological purpose, and we shouldn't ignore that, but the vast majority isn't irreplaceable.
There's no reason to believe that newly created (ecologically engineered) coast wouldn't be just as, if not more, biologically productive than what is being replaced.
Sure, it's possible for people to create new and better habitat for endangered species. The problem is that we usually don't. The "make more land" article doesn't even offer that as a suggested environmental offset. I imagine that the cost of doing so in a way that would actually be beneficial to the environment would nullify any gains to the developers.
I agree that coastlines aren't fragile on geologic time scales. The living things which depend on them are. There are millions of species that we haven't even cataloged, so how many are going extinct because of human development? We'll never know.
Ecosystems take many years to develop and often host unique species that don’t just pack up their bags and move where we want them to. When habitat is lost, extinction occurs— maybe not all at once, and maybe not species that we know of, but habitat loss contributes significantly to extinction.