Fiberglass is pretty inert, right? It's also used for boat hulls. I wonder if turbine blades could be cut up and used for artificial reef construction the way decommissioned ships sometimes are. But perhaps the blades are lighter than water.
Some of it has to do with the depth: most life is in the 0-40’ zone. And the oxygen drops off quite a bit, which inhibits oxidation (rust). There’s another wreck we dive at 40’ that’s barely recognizable due to corrosion and life. There’s an area that has coke bottles from 1945 (the glass is date-stamped) and they barely have anything growing on them.
If you dump fiberglass in shallow water, people are going to be pissed. If you dump fiberglass in deep water, it will likely not decompose for millennia. But it also won’t be a Mecca for perch and sharks like some make them out to be.