Two shoes worn equally can reach very different ends: one resoled several times over a decade, the other discarded with an intact upper. The sole material and how it is attached decide which happens.
Leather wears by abrasion alone
A leather sole thins gradually as it abrades against pavement, and the wear is concentrated at the ball and the heel where pressure is highest.
The material itself does not change chemically in any meaningful way, so a sole with thickness remaining performs exactly as it did when new.
That predictability is why leather soles are monitored by thickness. There is a point at which they must be replaced, and it arrives visibly rather than suddenly.
Welted construction makes replacement routine
In a welted shoe the upper is stitched to a strip of leather, and the sole is stitched to that strip rather than to the upper itself.
A resoler cuts the outer stitching, removes the sole and stitches a new one to the same welt, leaving the upper and its lasting untouched.
The process can be repeated several times, and the limiting factor is eventually the welt or the upper rather than the number of soles fitted.
Foam midsoles degrade whether worn or not
Cushioning foams are gas-expanded polymers, and the cell structure that provides cushioning compresses permanently under repeated loading.
Certain polyurethane formulations also break down through reaction with moisture in the air, a process that continues in storage.
This is why an unworn pair kept for many years can crumble on first use, and why sneaker cushioning has a functional life measured in time as well as distance.
Cemented construction resists repair
Most sneakers and many modern dress shoes are cemented, meaning the sole is bonded to the upper with adhesive rather than stitched.
Separating that bond without damaging the upper is difficult, and rebonding requires presses and adhesives that a general repairer may not hold.
Repair is possible on some constructions and uneconomic on most, which is the practical reason cemented footwear is usually replaced rather than restored.
Rubber sits between the two extremes
Solid rubber soles resist abrasion far better than leather and do not compress like foam, so they last considerably longer in ordinary wear.
Rubber does harden and lose grip with age as plasticisers migrate out, which is a slow change that shows first as reduced traction on wet surfaces.
Where a rubber sole is stitched rather than glued, it combines the durability of the material with the repairability of the construction, which is why that combination appears on boots intended for long service.