Interesting - assuming the panels last twice as long also drops the per-watt price pretty dramatically, which changes the economics on solar panels pretty favorably.
Panels come with 20/25 year warranty that after that period they have declined a maximum of 20% in performance.
It doesn't change the per-watt price at all and it might even increase the per-watt price as the average efficiency will likely decrease over its lifespan. It does drop the per-joule price though as it will produce more energy due to having a longer lifespan.