I am sure the alternator failures are caused by using the wrong oil. You know Dino oil does not carry away heat as well as synthetic, and if you put BMW on the can, it is even better.
Seriously, the varnish can darken and the wires can still be good. A heavy varnish coat can contribute to heat build up.
When an air cooled alternator stator goes bad you can hear a whine from the shorted turns. I wonder if anyone has noticed a difference in sound before a failure.
those who get them rewound, as for the highest temp magnet wire you can get, It does cost extra.A single thin dip of vanish is all you want.