They essentially last 50+ years now, and have always, barring damage. State of the art panels today warranty 90%+ output after 25 years. So they probably aren't far off what you're asking for at 50. They keep getting better and cheaper every year.
https://www.nrel.gov/docs/fy22osti/81172.pdf#page=5
Some types of panels have increased capacity after being in the field for years. Degradation is not a significant economic concern at this stage of the process.
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.
Many regions of the world are getting dryer. You might just be seeing a global warming side effect as there is less cloud cover.
* 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.
> For normal incidence, approximately 4 % of the light is reflected; this value is determined by the refractive index of the glass. For most glasses with a refractive index of 1.5, reflection losses at the surface result in an approximate 4% decrease in light intensity.
But I would bet it is cheaper to buy insurance for such infrequent events.