Loose spokes move microscopically with every pedal stroke. Moving spokes fatigue or literally wear out if the movement is between, for example, a steel hub flange and a spoke elbow.
Cheap spokes tend to die sooner.
So run tight wheels, or use decent spokes. Or both.
"Loose spokes move microscopically with every pedal stroke."
There is no relative movement between spokes and spokes, or spokes and other wheel parts during a wheel's use that's enough to cause any sort of wear in a spoke. Wear implies loss of material, of which there is none.
Spokes do flex during wheel revolutions and they flex at the bottom of the wheel in the load affected zone, which is an area between the road and rim. Even then, they only flex by becoming SHORTER. This change in length is indeed microscopic and it is taken up evenly along the entire length of the spoke. Therefore the change in length per mm of spoke length is indeed microscopic. For all intends and purposes the relative movement between hub and spoke, where the spoke touches the hub flange is thus zero. We know this because there is no visible rouge in that area when you disassemble a wheel, nor is there ever any signs of material loss on the spoke itself. The hub flexes with the spoke, reducing relative movement between the two.
Steel hubs with sharp edges do indeed cause more spoke breakages but this is not because of wear, but because of conditions that exacerbates metal fatigue. An aluminium hub, being softer than the spoke, changes shape to accommodate the spoke elbow. This smearing creates a larger contact area and softer bend in the spoke, which reduces work hardening and subsequently, metal fatigue in the spoke. Steel hubs don't, causing a very sharp transition at the spoke elbow.
Flex is not movement.
Spokes flex when just rolling along and additionally with each pedal stroke. Flex from the latter is confided to rear wheels and mostly the right hand spokes only and, only every second one on the right hand side. This flex is small since even a strong launch to sprint only increases the spoke tension in those spokes by less than 5% It is less than the flex induced by mere rolling.
"Moving spokes fatigue or literally wear out if the movement is between, for example, a steel hub flange and a spoke elbow"
Movement is not flex. A spoke that's moving is one that's rattling away in the wheel because it has no tension. Such a spoke experiences no forces which cause metal fatigue. As soon as a spoke has some modicum of tension, it will flex as described above and start to fatigue. The rate of fatigue is irrelevant whether the spokes are high tensioned or just-just tensioned enough to keep the wheel together.
I've dealt with the topic of wear. It implies relative movement, abrasion and loss of material. That doesn't happen.
"Cheap spokes tend to die sooner."
Price is not a reliable indicated of poor spokes, but it has bearing. It costs more to make a double-butted spoke than a straight-gauge spoke and therefore it should cost more. But a double-butted spoke from a SAPIM box or a double-butted spoke from a Campagnolo catalogue are exactly the same spoke, although the price is hugely different. I'm sure you know this but someone else may not.
Good spokes are made from de-gassed, 18/8, cold-forged stainless steel. Rubbish spokes are made from high-tensile steel and then galvanised.
Budget spokes are made from the same material as above, but not butted (drawn) and thus don't receive as much of the benefitical cold forging. But non-butted bit of the spoke, i.e. the thin shank, flexes more than the thick bits at either end. This reduces the strain on those bits and hence, inputs less metal fatigue in those critical parts. Metal fatigue is not as prevalent in the thin, flexible central shank even though it flexes more than the thicker parts.
However, an expensive spoke still needs one bit of material treatment to make it long-lasting and that treatment is stress relieving. It is done by the builder after the build. Unfortunately the spoke company can't build it in, it has to be done after the wheel is finished. Most builders don't have a clue about this, hence the poor wheels we see.
"So run tight wheels, or use decent spokes. Or both."
For long spoke life the wheels don't have to be tight. I explained why in the text above. Tight is good however, because it prevents nipples coming loose and wheels losing trueness. Very tight is even better, but it has nothing to do with spoke life. Uneven tension also has nothing to do with spoke life but you don't want such a wheel 'cause it wobbles.
