Solar panels can last 50 years. The main reason they don't is that they improve so much every decade that the opportunity cost of doubling power output per square meter makes it worth replacing them.

So you're left with 2 theses:

1: solar panels will stop improving, so you should amortize over 50 years. 2: solar panels will keep improving, so cost / kWh will be less than a 50 year amortization.