Explain that the Code is continually revised (on a 3 years cycle) by electrical experts working with the National Fire Protection Association, based on real-world experience with installations similar to what they have in their occupancy,
(OK - bonus if you can say the whole thing with a straight face
LOL - Code Committees = reality - ROFL)
and any upgrade / rewires you're recommending are for their safety and the eventual sale-ability of their property.
You can also explain (or try) that there isn't a "length of service" guarantee/warantee by the manufacturer of any type of electrical product that extends to 20,30,40 years or so...
Of course, when in doubt, yoou can always revert to:
Just because it worked doesn't mean it's safe.