Take the Disneyland Sky Ride for example. When this attraction was installed in the mid-1950s, it was typical of the wire rope cableway transportation technology of those times. Design and operating standards become ever more strict over time. This makes it tough to upgrade old equipment to the latest standards each time the rules change. At some point in time, some items of equipment become either uneconomical, or physically impossible to upgrade.
Another very real consideration is the wear and tear on machinery over the years. Many times the service replacement parts are no longer available. The existing parts have to be modified to accommodate different service parts (like bearings and such) and this kind of revision work can get pretty complicated. Pretty soon the basic framework parts begin to reach their ultimate fatigue life and require ever closer periodic inspections.
OK....Time out for Mechanical Reality 101. Ultimate fatigue life? Most materials, such as steel and aluminum, have a characteristic where as you apply more load and number of working cycles, the material will start out with an unlimited life span, but get shorter and shorter as the load increases. Everyone knows the "bend the paper clip trick". Bend it just a tiny bit back and forth and you might never hurt it. Now bend it back and forth harder...it soon breaks. That's a demonstration of fatigue life.
The trick in engineering anything is to make it heavy enough to last forever, but light enough to do the job you need done. Heavy as in, say railroad cars, light as in airplanes. Not all materials behave the same. Some mild steels have average strength so you don't expect to make them carry big loads. Certain aluminum alloys for example are very strong, and can carry big loads as long as you limit how many hours or years it will safely carry the load before the fatigue cracking starts. Now you can appreciate that not everything will last forever if it has to be light enough do a special job. Things that fly, or have to be very light will ultimately be life-limited in service. That's what we techno-heads call fatigue life limited. The Sky Way had a bit of that in it's original design when the Von Roll Company built it for Disneyland.
There are other special considerations too. We learned way early that manufacturers that supplied parts to Disneyland found their stuff to have a shorter life in Disneyland service. Wow, you want to improve your product?...take it to Disneyland. So we soon codeveloped a lot of components that worked super after a few years of understanding the severe service requirements on Disneyland attractions.
In the specific case of the Von Roll Sky Ride, we made numerous upgrades over the years to stay ahead of the severe service wear and tear. But eventually the combination of fatigue cracks, can't get service parts, much tighter new service standards...our wonderful old Sky Ride became a cranky old senior and had to go to the Extinct retirement home.