Here is a quote about spark plug electrode wear:
"The electrode wears from the locations that discharge easily with spark discharge.
In particular, since the center electrode reaches high temperatures, it oxidizes and wears.
The amount of electrode wear varies with the electrode material melting point, strength, hardness, etc. In order to reduce the amount of this wear, nickel alloys, platinum, iridium, and other such materials are used for the electrodes and service lives are also extended with fine electrodes."
Here is a quote about the "air" gap affects:
"The required voltage (the voltage required to discharge across the gap) rises in proportion to the distance driven.
This rise in the required voltage is large until the sharp section at the end of the center electrode is worn round to some degree (about 4,000km). After that, the main factor is the enlargement of the gap due to electrode wear and the rise in the required voltage is smaller."