It just feels like these structures were built to last a lifetime and then some. Most of the stuff being built nowadays appears as though it already has a due date of "the not to distant future" to be torn down and replaced.
At least here in the US, the architecture just feels like everything else around me - it's disposable.
For a lot of infrastructure, the ability to more accurately model and calculate the performance of structures has meant that they don't need to be overbuilt so much. The less you overbuild a structure, the less it weighs, and the less structure you need to build - a virtuous cycle.
If you look at the last 400 years of bridge design you can see the transition from overbuilt compression structures to gossamer suspension bridges. This is a good thing, but it's easy to mistake old, overbuilt structures as being more "robust" when what they really are is an inefficient design meant to avoid failures due to a weak understanding of how the materials and structure will perform under use.
The thing is, their goal wasn't to last for centuries. Their goal was just to not fall down, and because people back then didn't have the modeling abilities we have now, their solution was to be very conservative in their design and overbuild everything. If we wanted to make bridges that would last a long time, we could almost certainly do it more efficiently now.
Yes, stone will last longer, simply because it doesn't get eroded by the elements as quickly as possible. But it's also simply not usable as a construction material for bridges beyond a certain size and span, because it's only good under compression and even then its own weight starts to become a limiting factor as you get larger.
By the 1940s, a stone bridge that might take 100 man-years of labor to create could be replaced by a bailey bridge that goes into place overnight. I'm sure if we wanted to create a bridge that lasts 300 years, we could dedicate more resources to it, but it would be grossly wasteful to do so - you don't know how long the current requirements are going to even be relevant. It's better to build a bridge that lasts for 50-60 years, and have the option value of replacing it with a better structure down the road.