Just bake new panels into the cost of a new roof. It's probably time for both after 25 years.

This is standard now. My REC Alpha Pure Black 400 panels are warrantied for 92% at 25 years.

The degradation is about 0.25%/yr. Assuming no exponential drop-off it would take >50 years to hit 80%.

Essentially no one has had to replace their panels due to degradation losses as of yet (since the vast majority of panels have been installed in the past ~30 years).

The idea that PV panels become trash in a decade is a lie promulgated by oil companies. According to NREL, reputable panels degrade as little as 0.1% per year, which means they will still be above 90% of rated output after 100 years. The trashiest panels NREL has tested degrade at 1.1% per year which would put them at 75% rated capacity in 25 years.

https://www.nrel.gov/pv/lifetime.html

Generally you can find the degradation curve in their datasheet, and yes, 80% after 25 years sounds possible. IIRC they fade faster with time, so it might be 50% after 50, but ... that's still pretty good.

you'll need to supplement with some new panels to account for the loss, but each new panel is more efficient than the one you're "replacing"