aside from the points already made about if the customer has asked for it to be ripped out i think you need to make a judgment.
you state the IR's between conductors are above 300M ohms which i would consider as excellent readings. also what are the cable like visually? junks missing anywhere? glanded ok? any signs of rusting on the armour?
all things you should be considering when assessing whether its fit for use. the factory where i work has many swa cables probably older than what you have running submains ect. when i work on the circuits i simply cant justify replacement of the cable on age alone. replacement of some of these cables would cost thousands of pounds and would be an absolute nightmare to run in. i prefer a process of regular testing and inspecting to assess condition and replace if i think its required.
why not contact manufacturers if they are still going, they may offer advice, they may not. but 20 year old cable may still be perfectly fine to leave in place, just depends how hard a life its had.