I kept the same phone for five years, which everybody told me was unreasonable. It cost me two battery replacements and one screen, and it was working perfectly well when I finally replaced it.
Here is what actually degraded over that period, which turned out to be a shorter list than I expected.
The battery is the whole problem
Almost every complaint people have about an ageing phone traces back to this, and most people do not know it.
Lithium cells lose capacity with charge cycles. After two or three years of daily charging, a battery typically holds meaningfully less than when new, and the decline continues.
What that produces is not simply shorter runtime. Below a certain state of health, the phone begins to throttle performance deliberately, because a degraded battery cannot deliver current fast enough for peak demand without the device shutting down.
So the phone becomes slow, and the owner concludes it is old and needs replacing. It needs a battery, which costs a fraction of a phone.
Both of my replacements produced a device that felt new. Not slightly better — indistinguishable from new in everyday use.
What did not degrade
The processor, which does not wear out in any way a user perceives.
The screen, until I broke it, and the replacement was fine.
The cameras, which produced the same images in year five as in year one, because the sensor and lens do not age.
The storage, which has a finite write life that is far beyond typical phone usage.
Which means that on a five year old phone, essentially every component except the battery is performing as it did at purchase.
What genuinely gets worse
Being fair, some things do.
Software support ends, which is the real constraint and the one worth planning around. Once security updates stop, the calculation changes, and this varies enormously between manufacturers.
Applications become more demanding over time and run less smoothly on older hardware, though this happens more slowly than people assume.
New network features arrive that older hardware does not support.
And accessories move on, particularly connectors, which is an artificial constraint rather than a technical one.
The repair experience
Worth describing because it is the practical obstacle.
The battery replacements cost a modest amount at an independent repair shop and took under an hour each.
The manufacturer route was several times the price with a longer turnaround, and in one case I was told the device was too old to service.
Independent repair is legal, widely available, and manufacturers have historically made it difficult through adhesive construction, proprietary fasteners and parts pairing that produces warnings or disables features when a component is replaced with a non-original one.
That situation has improved somewhat under regulatory pressure in several jurisdictions, with parts and manuals becoming more available, and it is still worse than it should be.
What I would check before buying, now
The criteria that would have saved me some trouble.
The stated software support period, which manufacturers now publish and which varies from two years to seven. This is the single most important specification and it appears on no comparison chart.
Whether the battery is replaceable at reasonable cost, and whether parts are available independently.
Repairability ratings, which exist in some markets and are becoming mandatory in others.
And whether the device supports the current network standards well, since that is a genuine obsolescence path.
None of those are the things reviews lead with, which tend to be camera comparisons and benchmark scores that stop mattering within a year.
The financial arithmetic
Straightforwardly, keeping a phone five years rather than two costs roughly a third as much per year, plus perhaps two battery replacements.
Against that, you are running older hardware and eventually unsupported software.
The point at which I would replace is when security updates end, and not before, unless something breaks in a way that is uneconomic to repair.
That is a considerably longer cycle than most people run and considerably shorter than the hardware is capable of, which is the frustrating part.
The thing I would tell somebody
If your phone feels slow and it is more than two years old, replace the battery before replacing the phone.
It costs very little, it takes an hour, and in my experience and in most accounts I have read, it resolves the problem completely.
If it does not, you have lost a small amount and learned something. If it does, you have deferred a large purchase by two or three years, which is a better return than almost anything else available.
The trade-in comparison
One thing worth putting against the repair argument.
Manufacturers and networks offer trade-in values that fall sharply with age, so a phone kept five years is worth very little when you finally replace it, whereas one traded at two years retains meaningful value.
That reduces the saving from a long hold, and it does not eliminate it. Running the numbers on my own case, the two-year cycle with trade-in was still substantially more expensive per year than five years plus two batteries.
It is worth doing that arithmetic for your own situation rather than assuming either way, because trade-in values vary enormously by manufacturer and the gap has narrowed on some models.