I have a problem understanding your logic the "conducted through the insulation" because if it is 60, 75, 90, 105 deg there wouldn't be any different and is a mute point.
It is not moot. The insulation on wire is designed to insulate against the flow of electrons from one side to the other. It does not insulate particularly well against heat, although it is still a lot poorer conductor of heat than the copper metal.
But the heat that goes out through the copper will end up somewhere, and that somewhere, for a long wire at least, will be conducted out through the insulation. The reason that when multiple wires are in the same raceway the same current (and therefore I
2R power) will cause those wires to get hotter. That is because the heat coming from the wires through the insulation is trapped inside the raceway until it is conducted out through the walls of the raceway. That will happen more slowly than if the same insulated wire were in free (convecting) air.
Now, as in pretty much all things thermodynamic, the amount of heat moving from one body to another will be proportional to the temperature difference between them (although not necessarily linearly when convection comes into play.)
The higher the wire temperature, the faster it loses heat through the insulation and the higher the temperature of the outside of the wire, the higher the conduction and convection of heat away from the insulation.
So a wire whose insulation can tolerate a higher temperature will be able to give up heat to its surroundings better.
Fixture wire that can tolerate 100C will be able to carry more current without damage than that same copper limited to 60C. And that wire will be losing heat through the insulation to a 30C ambient very roughly 70/30 as rapidly per unit length.
(At any given amperage, the wire will heat up until the point where the heat losses exactly match the I
2R heat creation. The hotter that temperature can go, the more current the wire can safely carry. That is why 18AWG 100C wire can safely carry more current than 18AWG 60C wire.)
Executive summary: When you overheat the fixture by putting in too large a bulb, you are better off with higher temperature fixture wire, up to and somewhat beyond the rated termination temperature of the socket, etc.