In the case of the 2 farmers I know I am almost certain that in the unlikely scenario that they became responsible for the removal of old wind turbines they would respond "Bugger that, they can stay there". Seriously though they have sites that are ideal for wind power, if the current turbine owner jumps ship, goes broke or just reneges on the lease then there will be someone else willing to take over the sites and it would be up to them to remove the old turbine or keep using it.
I assume you are saying that primary production is a dicey industry, that would be correct but the wind turbine leases are a very stable, reliable, indexed income with no input costs, completely the opposite to primary production. In a bad year they would be a major proportion of income, in a good year they will still be very handy pocket money
