This is utterly wrong. The brake pads cover less than one square inch of the rim. They're not some blanket that prevents heat from escaping. Your rims are losing heat at the same rate regardless of whether the brake is on or off. You don't need to "get off the brakes" so they can start cooling. They are already cooling, all the time. All this talk about the surface of the rim vs the core of the rim is just a bunch of mumbo jumbo. Metal is an excellent conductor of heat. Aluminum is used as a heatsink in many applications. The heat is distributed throughout.
A “heatsink” is just what it sounds like…a place to sink heat into. It can be a heat exchanger but not necessarily. Metal can “sink” (or absorb) a whole lot of heat because of the metal’s high melt point. Some heat will radiate away from the metal if the metal isn’t insulated but if you keep putting heat into the metal, it will just keep absorbing the heat to the point where it eventually melts. Granted that takes an enormous amount if heat but, for the purposes of this discussion, the more fiction your apply to the metal, the hotter it will get and the hotter it will stay…i.e. you “sink” more heat into the system.
“Getting off the brakes” or not dragging the brakes, if you like, doesn’t constantly “sink” heat into the rims like constant fiction does. The whole point of stopping in the middle of a downhill to let your brakes cool is to stop the friction on the metal so that it has a chance to radiate away. The same thing is accomplished by pulse braking without the bother of stopping. In fact, the heat exchange with the air (different from the heat sink) is more efficient when air is flowing over the rims which is what makes pulse braking more effective than constant braking with a stop to cool the rim.
Constant vs non-constant braking is also meaningless. It depends on how hard you're applying the brakes. Friction = coefficient x normal force. If you only lightly feather the brakes, there will be hardly any friction. You can apply the brake continuously for 8 hours straight and your rims will still be cool as a cucumber.
Yes, that is the equation for friction. But there is more to the story than just the amount of friction. There is the rate at which the friction is applied and the rate at which it can be radiated away. You’ve already addressed that above with your comment about medium speed resulting in hotter the wheels will become. Part of that is because you can’t reach high speeds to take advantage of the greater air flow if you constantly use the brakes to keep your speed low. Attaining high speed is only possible if you use the brakes sparingly…i.e. with pulsed braking.
Keeping a high speed in the straights and braking at corners has nothing to do with getting off the brakes so they can "start cooling". Your rims are always cooling regardless. All you're doing is increasing your average speed and thereby increasing the percentage of energy converted via air friction vs via brake friction. Less energy going into the brakes = less rim heat. The heat that would have gone into your rims went into your face and body skin instead.
If your brakes will stay cool with constant braking, then why do people who practice that type of braking need to stop in the middle of a descent to let their brakes cool. A 3 to 5 mile downhill isn’t all that long. What you describe…braking hard for corners and letting the bike run without brakes in the straights…is the definition of “pulse braking”.