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.
* 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.
Many regions of the world are getting dryer. You might just be seeing a global warming side effect as there is less cloud cover.
But I would bet it is cheaper to buy insurance for such infrequent events.
> 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.