I'm not sure how these 2 statements are remotely related. Subways like anything else still need to be maintained, and failure to do so will lead to problems. Rails of any kind can be seen as a consumable good - you can only run so many trains on a piece of track before it needs to be replaced. This is true for streetcars, lightrail, subways, mainline, whatever.
When people claim that "Subways last 100 years", they're usually specifically referring to the tunnels, which generally can last 100 years with only light maintenance (not to say early refurbishments can't or shouldn't be done for a variety of reasons, whether if its to flood proof, seismically harden, or remove asbestos like the TTC did a few years back). However once you move past the actual tunnels and infrastructure specific to subways, they need to be treated like any other railway. In fact, with a subway network like what we have in Toronto, you frankly need even stricter maintenance since we're running massive 145m long, 3.3m wide trains as frequently as every 2 minutes - meaning the wear and tear on the tracks will happen much quicker compared to say a GO train line. The only saving grace is that much of the network is underground, so its generally immune to the elements. However that doesn't stop the above ground sections of the system being hit hard whenever there's a heat wave like what we had 2 weeks ago (not coincidentally, that's likely what started this recent wave of TSRs)
