There are several 50 year old solar installations that are still working fine. They've only degraded 10%-20% or so. The reason that they are typically replaced after ~20 years is that the technology has gotten so much better that you can replace the old panels with new ones and get 4X the power in the same space. The wires and the mounts are worth more than the panels, so reuse those with modern panels.
Considering they work by basically taking electrons from the material, they are guaranteed to get much less efficient and I doubt it's a linear effect.
I think once you have destroyed the first layers, it becomes much more complicated to get meaningful power depending on variables.
Solar panel talk always focuses on the ideal conditions like California, but you have to account that plenty the energy transition is necessary in place where solar panels are not that efficient to begin with.
