I agree with the statement that fiberglass hulls themselves are pretty ecologically friendly. To a certain extent I was playing devil's advocate; I think the whole project is not really sustainable and so I'm picking on certain aspects.
I get a little bit up in arms when people propose ideas promoting "sustainability" without really addressing what sustainable means. Pouring thousands of dollars every year (plus a million dollar initial cost!) into a toy is not a sustainable use of resources. Unless the OPs come up with some sort of economically viable use for their 50-foot wing-sailed multihull yacht, I think it's borderline immoral to call it sustainable.
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27