And that's exactly how an investor would figure the payback period of a wind farm.
Also, check Vestas corp that makes them. Hybrid metals, exotic resins, plastics, carbon fiber composites, not wood and sailcloth any more.
Modern wind mills are not left to rust, nor are they buried in the ground after their useful life (estimated to be 20+ years). Most wind farm leases and local permits require dismantling, removal and site reclaimation provisions. And many old wind farms can also be "re-powered" with the newer, more efficient machines, which involves the removal and disposal of older machines.
No forms of modern power generation is 100% bio-degradable, nor without some form of "subsidy," so holding windmills to such a standard is a strawman argument.
How much land is used up by a wind farm? Sounds like a lot.