I guess I'll chime in; I had to think about what I would say.
I think the price is outrageous, I'm also not surprised at the price.
You've got a lot of red flags right from the start: Northern California, and hilly.
So, earthquake potential, expensive real estate, lots of bureaucracy, maybe even litigious neighbors?
As a contractor, or a structural engineer, there is tons of liability here and you can be sure they want to cover their a**.
But the structure you're describing does not sound very complicated to justify such an extreme price.
I agree that it might be possible to think of this as a repair, or maintenance, and simply replace what needs replacing, possibly with some upgrades, as long as the footings were substantial and still intact with no settling. Twenty years old? Do you have the original plans, and the original building permit? Was the deck originally built under the building permit for the house or was it added afterwards? If it was inspected and approved twenty years ago I would feel better about just rebuilding it now.
I also agree with Dicey that how the posts are attached is important. It's also important that the deck ledger is securely attached to the framing. I use deck ledger screws. A good lumber yard with a Simpson catalog (maker of metal anchors, hold downs, straps, etc, all stuff for earthquake country) can point you in the right direction here, they sell the stuff every day.
Also , where are the posts rotting? They shouldn't be rotting at this point, in my opinion, so that worries me that the deck may have not been built (or designed?) well from the start.