I believe the reason that the wick degrades at ‘just the right rate’ is that the flame actually keeps oxygen from reaching the wick (so it can’t itself burn) until the level of wax goes down enough that the top of the wick pokes out of the now lower flame.
The wick itself is not what’s burning, it ‘wicks’ (draws up) the liquid wax up to the top where it evaporates and burns as the flame.
The heat does eventually degrade the wick, just at the right rate to stay close enough to the top of the wax to melt it.
I believe the reason that the wick degrades at ‘just the right rate’ is that the flame actually keeps oxygen from reaching the wick (so it can’t itself burn) until the level of wax goes down enough that the top of the wick pokes out of the now lower flame.