I have never seen a panel which had less than a 20 year guarantee to be working and producing 80% of intial power at that point. Many had a 25 year. Nothing was less than 20 though, so I'm curious where the 1-5 year figure comes from.
As for other generators, yes, large scale solar and wind are great - and of course CA has many of both. Rooftop solar is one more tool in the arsenal.
I think my assumption, generally, is 1% annual degradation. However, to be more pointed in my response: I see a hell of a lot of panels sold by Chinese companies that I'd bet won't be around in 20 years, or if they are, good luck collecting on that guarantee from a Chinese-owned, state-backed entity in the mainland. Yeah, no.
Edit: the implication of which is they can promise whatever they want.