One can't argue with that. However, as with the case of the rodent damage illustrated above, it is unfortunately the case that, if the circuit were 'working' at the time it is probable that no amount of testing would have revealed any problem.That's the reason we recommend having your electrical installation inspected and tested every 5 years. You were very fortunate it did not cause a fire.
The rodent damage I illustrated was found purely by accident when a floorboard was lifted (for reasons unrelated to the electrical installation). When I saw the damage, I IR'd the circuit and, as one might expect from the appearance of the damage, the result was totally fine (since the exposed bare conductors were all separated by 'air').
Kind Regards, John