>> 1MW of solar produces 1MW in years 1 through 20.

I agree with your post, but the above statement is not technically accurate.

Panels do drop in production over time; A very rough rule-of-thumb is around 1% loss per year.

So after 20 years production is at like 80%. (Ymmv)

Of course this isn't really a big deal. The panels themselves are cheap and can be replaced very easily. Equally there is a market for 2nd hand panels as they still produce, and are ideal for places that are not space constrained.