500N - Concrete is basically inert and doesn't break down for hundreds of years at the minimum. What happens to concrete when it fails, is the reinforcing bar corrodes, the growth of the corrosion produces expansion stresses, and the concrete fractures.
It needs moisture ingress for this to happen, and properly constructed concrete structures are designed to stop moisture ingress into the reinforcing bar.
There are still tens of thousands of abandoned concrete structures, such as WW2 fortifications, that pose no threat to anyone. Concrete is 100% recyclable and the reinforcing bar can be recovered and smelted into new rebar.
End of life recycling
I agree that the windfarms should have environmental bonds posted, to cover cleanup costs when they are finally abandoned.
Currently, EPA approvals for windfarms outline cleanup guidelines, and the lease agreements with landowners normally outline precise cleanup procedures upon the termination of the project.
However, it's not like there's massive costs involved, nor are there any contentious/dangerous materials involved in abandoned windfarm residues.
We have an abandoned windfarm here in W.A. (Salmon Beach windfarm), it was built in 1987 and the turbines reached their designed lifespan after 13 yrs, and they were replaced by superior designs in another position.
Urban encroachment was another reason for the closure of the Salmon Beach windfarm.
The Salmon Beach windfarm was a total success at the end of the 13 yrs, despite the fact it used early-design, W.A.-made wind turbines.
The abandoned windfarm site was cleaned up according to the requirements of the Clean Energy Council, and no-one has complained about the site, or any potential residual health hazards emanating from it - because there are none. The old windfarm site is now actually a tourist trail.
Abandoned windfarms, cleaned up to the regulatory standards, pose no threat to anyone - unlike the 40% of Australian homes and buildings that still contain sizeable amounts of asbestos.
If you need something to worry about, worry about that asbestos - not harmless concrete foundations of abandoned windfarms - which can be 100% recycled with ease, anytime it is so desired.
Fact sheet - Decommissioning Wind Turbines