A well-made leather bag often ends its useful life because a zip has broken or a strap ring has worn through. The leather is usually fine. The reason is that hardware carries load in a fundamentally different way.
Load concentrates at attachment points
Weight inside a bag is distributed across the whole base and body, so the leather experiences low stress spread over a large area.
All of that weight then passes through a small number of rings, rivets and stitched tabs to reach the strap, concentrating force into a few square millimetres.
Materials fail where stress is highest, so the point where a strap meets the body is the predictable failure site regardless of how good the leather is.
Moving parts wear rather than age
Leather changes slowly through flexing, drying and abrasion, and it can be conditioned and repaired along the way.
A zip, a clasp or a swivel hook is a mechanism with surfaces sliding against each other thousands of times, and every cycle removes a small amount of material.
That wear is cumulative and irreversible. Once tolerances open up, a zip skips its teeth and a hook no longer closes securely.
Plating is thin by design
Most bag hardware is a base metal, often brass or zinc alloy, finished with a thin plated layer of gold tone, silver tone or a lacquer.
The plating provides colour and corrosion resistance and is measured in microns, so friction at contact points removes it comparatively quickly.
Once it is gone the base metal is exposed, which discolours and can corrode, and the visible tarnish often prompts replacement long before mechanical failure.
Zips are the most complex component present
A zip has two rows of teeth, a slider that forces them together at a precise angle, and stops at each end, all working within tight tolerances.
Failures usually occur in the slider rather than the teeth, because the slider does the work and wears at its narrow throat.
This is why a zip that stops closing can frequently be repaired by replacing the slider alone, provided the teeth themselves are undamaged and aligned.
Repairability is decided at manufacture
Hardware attached with screws or through replaceable stitched tabs can be swapped by a repairer with ordinary tools.
Hardware riveted through a lined panel, or a zip stitched into a bound seam, requires partial deconstruction of the bag, which multiplies the labour cost.
Houses that offer long repair programmes tend to specify hardware they can source for decades and attach it in ways that allow removal, which is a real difference from bags built to be assembled only once.