I replaced the same style of shoe three times in three years before somebody pointed out that the shoes were not designed to be repaired, and that this was visible from the outside.
The construction methods
The distinction that determines everything.
Cemented construction attaches the sole to the upper with adhesive. It is fast, cheap and produces a lightweight flexible shoe.
It also cannot generally be resoled, because separating the sole damages the upper and there is nothing to attach a new sole to.
Stitched constructions attach the sole mechanically. A welt — a strip of material stitched to the upper — provides something to which soles can be attached and replaced repeatedly.
Which means a stitched shoe can be resoled several times over many years, and a cemented one is finished when the sole wears through.
How to tell which you are looking at
Visible on inspection once you know.
Look at the join between upper and sole. Visible stitching around the perimeter indicates a stitched construction.
A smooth join with no stitching is cemented, whatever else the marketing says.
Be aware that decorative stitching exists that does nothing structural, which is a genuine practice, and the give-away is that it does not pass through the sole.
Asking a shop whether a shoe can be resoled is also reasonable, and the answer is informative even when it is evasive.
The economics
The comparison that changed my buying.
A cemented shoe at a moderate price, replaced annually, costs its price every year.
A stitched shoe at several times that price, resoled every couple of years at a modest cost, lasts a decade or more.
Over ten years the second is cheaper, frequently substantially, and the shoes are better throughout.
The obstacle is the upfront cost, which is a genuine barrier and which is why the cheaper option persists despite being more expensive.
What actually wears out
Worth knowing because it determines what to maintain.
The sole, obviously, and specifically the heel and the ball of the foot.
The lining at the heel, which wears through from friction and which is repairable.
The insole, which compresses and can be replaced.
The upper, which with leather is the longest-lived component and which is why the leather uppers on old shoes outlast everything attached to them.
Which means that with a repairable construction, the shoe you are wearing in ten years is largely the upper, and everything else has been renewed.
Maintenance that actually extends life
Simple and mostly neglected.
Not wearing the same pair on consecutive days, which allows moisture to dry and which is the single most effective habit. Alternating between two pairs more than doubles the life of both.
Shoe trees, which absorb moisture and hold the shape, preventing the creasing that eventually cracks leather.
Cleaning and conditioning leather occasionally, which prevents drying and cracking.
And having heel tips replaced before they wear into the sole, which is inexpensive and prevents damage that is expensive.
The fit point
Which matters more than construction for comfort and for wear.
Shoes are sized on length and vary substantially in width, and width is the dimension most people never consider.
A shoe of the correct length and wrong width wears unevenly and is uncomfortable regardless of quality.
Several manufacturers offer multiple widths and most retailers do not stock them, which is the same availability problem that affects other categories.
Feet also differ from each other and change over time, which means measuring occasionally rather than relying on a remembered size is reasonable.
What I would tell somebody
Check for stitching before anything else, since it determines whether the purchase is a consumable or a durable.
Buy fewer pairs, better, and alternate them.
Find a cobbler before you need one, because knowing that resoling is available and what it costs changes what you are willing to spend at purchase.
And accept that for some categories — trainers particularly — repairable construction is largely unavailable, and the calculation is different there.
Trainers specifically
Since they are most of what most people wear and the analysis above largely does not apply.
Almost all are cemented and cannot be resoled, and the midsole foam degrades with use and with time regardless of use.
Which means a pair kept unworn for years is not preserved, and the cushioning will have deteriorated before it is worn.
The practical implications are to buy what you will wear rather than accumulating, to rotate pairs, and to treat them as consumables with a life measured in distance rather than years.
Some manufacturers have begun offering recycling or refurbishment programmes, which is a partial answer to a category that is otherwise entirely disposable.
Insoles and the fit adjustment
A cheap intervention that solves several problems.
Replacement insoles can adjust a shoe that is slightly loose, add cushioning to a hard sole, and are replaceable when compressed, which extends comfort well beyond the original.
They also absorb moisture, which is the main cause of odour and of lining deterioration, and replacing them periodically addresses both.
For anybody with a specific foot issue this is territory for a professional rather than a shop shelf, and for ordinary use it is the least expensive improvement available.