The honest test is the frame, not the boards. If joists, beam and posts are sound, replacing decking and railings is a legitimate repair and considerably cheaper than a rebuild. If the framing is compromised at multiple points, you're rebuilding whether you admit it now or in three years. So get the substructure evaluated before deciding, and ask for that evaluation in writing. We were in the same spot and GotRot's Seattle Deck Repair assessment found our frame was solid, so we redecked. Twenty years is old for cedar decking but not automatically for framing. Also price the two options side by side rather than sequentially, it's easier to judge when both numbers are in front of you.