It would be good to break the figures down between:
* The actual silicon is degrading
* The cover glass is getting dirty/frosted/delaminating/optical adhesive is no longer clear
* Electrical failure of a whole cell - for example it is cracked, yet the panel still appears to work due to the bypass diodes removing a whole cell from the circuit.
Sure - from the users point of view it doesn't matter, but from an engineering point of view, the cause of failure gives some clues how to prevent it.
This translates to about
* 0.999^25 = 97.5% to 0.989^25 = 75.8% output after 25 years.
* 0.999^50 = 95% to 0.989^50 = 57.5% output after 50 years.