[brians]

Capacitor electrolytics.

Air bubbles in silicon, thermal expansion, and a zillion day night cycles.

Humidity and dust enter, contributing to occasional momentary shorts.

Oxidation anywhere you have electromechanical connections.

Magnetic stresses on any coil, any rectifier.

[kazinator]

Transistor aging: https://spectrum.ieee.org/semiconductors/processors/transist...

(Not likely to affect anything you'd mount on a bicycle, over any span of time relevant to a bicycle.)