Fishing rods ain' t arrows. Arrows ain' t fishing rods. The stresses are totally different.

There is wear on the outside diameter of the shaft from going into and being pulled out of target materials. If you take a micrometer and measure the diameter of the first few inches of an arrow that has been shot a lot and compare that to the middle of the shaft, you' ll find a good bit of difference. I' ve got some carbons that show nearly .020" of wear.

Arrows get rapidly compressed, front to back, going into the target and stretched when being pulled out. The friction going into and coming back out of a foam target heats the shaft, and repeated heating and cooling does it' s own small bit of damage. Shooting them in full sun in extremely hot weather softens the epoxy matrix and changes their spine a bit. The epoxy gets harder and a tad brittle in extreme cold.

Compression, elongation, heat and cold cause the epoxy to break down which also changes the spine. The outside diameter wearing down changes spine. Not to mention the UV breakdown you pointed out.

I doubt you can find a single pro shooter, or even one of the better amateurs, that sticks with the same dozen arrows for an entire season of 3D. No matter what they' re made out of, ALL arrows lose spine. Some faster than others. Wood is worst. Carbon is better. Aluminum is best.

It' s not a myth. It' s a fact.