Proper maintenance being the key phrase, there’s no reason the monorails couldn’t last 100 years either (other than money), they’re the equivalent of a classic car at this point, they’ll last forever as long as they can find the replacement parts, don’t mind paying the premium price for those parts, and have the maintenance staff to maintain them.
They’ve gotta be near the cheaper to replace age though, I’ve replaced a few cars over the years because I was spending more every month keeping them running than the payment would be on a new car.