That's a lot of guessing though, newer panels might as well be more durable and longer lasting. Even if you lose 20% in 20-30 years you don't need to replace the panel unless the cost of replacing them can be recouped within a reasonable amount of years. As long as there is more space for more panels you don't need to replace existing ones unless they stop working, so capacity just increases for decades until you reach some saturation point.