When natural habit is destroyed by human development, it is not replaced by something "better". It is replaced by something different, which benefits some species over others (mainly humans). Coastlines and wetlands are never "moved", they're simply destroyed. The life that was there dies and species go extinct, one after the other.
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.
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.