The answers to your questions are in the 52-page Renewable Energy Foundation report “The Performance of Wind Farms in the United Kingdom and Denmark“. There’s a bit of math and statistics — the details are in Appendix: Data and Methods. It’s also worthwhile to study the full report I quoted Why is wind power so expensive? An economic analysis

For the United Kingdom the raw data were extracted from the Renewables and CHP Register database compiled by Ofgem. This government data is used in the administration of the market in Renewable Obligation Certificates (ROCs, otherwise known as transfers from taxpayers to wind operators).

The decline in operational performance is empirical fact. As I read the data the rate of degradation is not improving with newer generations. The technology is mature so I would expect initial performance to be worse as the best sites are populated first.

The causes of degradation from initial power-on are likely a combination of industrial aging (vibration which increases rate of bearing wear, etc), blade performance degradation due to chips, dings and accumulated coatings of bird and bat bits, and downtime. Usually aging equipment needs more servicing so you observe increasing outages of longer duration. Vestas and GE probably have good data on the performance degradation, but it isn’t obvious why they would be motivated to publicize what they know. They get paid at both ends, initial sale, replacement turbines and other parts.

I don’t know how much the operators care as the taxpayer subsidies are so high they make money no matter what. If the subsidies went away the whole “industry” would evaporate except special cases where the grid is distant from demand and the supply of high quality wind is close to site of demand. And there is nearby hydro for cheap storage.

It may well be that the output of wind farms is greatly reduced with age, as the article asserts, but the article neglected to provide the reasons for that. I am inclined to ignore articles that do not adequately back up their conclusions.

Is the problem that the permanent magnets lose strength as they age? Is it that the pitch control mechanism becomes arthritic through age and unable to maintain the optimal blade pitch? Is it that the exterior paint darkens through age thereby causing excessive solar heat gain requiring that the power output be reduced to prevent overheating? Is it because the tower becomes weaker because of corrosion thereby requiring that the blade pitch be reduced to reduce the wind load on the tower? It it because the surface of the blades becomes rough thereby reducing aerodynamic efficiency?

EXACTLY why does the output drop as aging occurs? It it a problem that could be rectified? Why are we just now learning about it? Could the problem have been anticipated?

Why did the article fail to offer adequate explanations?

Will there be a subsequent article that does offer adequate explanations?