Expecting "a couple of decades" from even the best two part polyurethane paints on a deck
would be highly optimistic, in my experience. Maybe in a cloudy climate, in a high latitude, hauled out and covered for half the year, but the tropical sun will take it's toll LONG before your paint job sees a twentieth birthday.
I agree that a 3 to 5X lifespan over a one pot paint is reasonable. But a FAR more realistic exception is 7 to 12 years from a two part paint with horizontal sun exposure.