I have seen equipments that came back 2-3 years later with the new replaced capacitors showing aging and needing replacement but the unreplaced old cap still measured well.
Probably because those which were not replaced were not under stress to begin with, unlike those which failed. Also capacitors are often replaced with wrong type. You shouldn't simply put anything even of top brand when replacing capacitors in places with high ripple current like in SMPS. And not all LOW ESR series are equal, rated ripple current can vary drastically.
Agreed. The failed cap that can affect operation are normally the stressed cap. The original old cap that failed lasted longer than my replaced cap. I don't have the data of how long it was running, my guess was it could have been 10+ or so years. Normally these are running 247 outdoor.