How to predict the shoreline rates to next 100 years?

“In the space of one hundred and seventy six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over a mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oölitic Silurian Period, just a million years ago next November, the Lower Mississippi was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-pole. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo [Illinois] and New Orleans will have joined their streets together and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.”

Life on the Mississippi (1884)

Extrapolation as you seem to wish to do is a random thing. If you fail to provide any viable model for the system (that includes known, highly important factors) you will be able to predict any result that you wish to see.

Extrapolation over a long term is a dangerous thing. Done without a valid model, a senseless task.