Not a physicist here, but does this aero drag really burn off that much heat for a bike rider descending like at 25mph vs 15mph? To the point that it will save that much heat?

I could speculate the converse 'extreme' case - that braking hard all the time (in a ridiculous way) might the safest, lowest temp way to go. Like. if you go do the Alpe de huez at a measly 5mph - the whole way down. I'll bet in that case the heat generated will be so low that you will never come close to blowing out a rim?

Years ago I made a chart of the amount of power the brakes needed to absorb vs rider speed. For a 10% grade the worst case was trying to hold a speed of about 35kph; go faster or slower and the braking power goes down. At terminal velocity braking power is zero.

I've had similar experiences to you but warped two fronts over the years descending from HC climbs at the TDF after a stage. So many people descending you have to ride the brakes all the way and both times felt like a buckle in the rim towards the bottom, rim destroyed but got me down. Once in the Alps over about 18km and Pyrenees 20km.

One of the reasons I switched to disk brakes!

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Don’t know the answer to your question but I irreparably warped a Zipp 404 years ago descending Whiteface Mountain. 8 miles of 8% grade that was beset with frost heaves and I had to brake heavily all the way or else I felt like I’d be bucked off the bike.

The heaves weren’t noticeable on the way up. Maybe because of how slow I was going? Ha.

For all you more knowledgable or speculative folks out there, is there any situation where excessive heat buildup is a problem on alloy-rim wheels? They don't even have to be carbon wheels, just curious.

Anytime you are having to dissipate that much heat you can have problems, though the situation where you could have issues becomes really rare.

The advantage of aluminum is that it is a much better conductor of heat and you will never hit a temperature where the structural properties will change dramatically like you can with the resin in a carbon rim.

In 30+ years of riding I have had one situation where I had heat problems with an aluminum rim. I was riding a hilly section of gravel road and when I came over the top of one hill I found that the county had put down a fresh layer of loose gravel on a ~1/2 mile decent that averages about 13%. I got down to about 15mph before I hit the gravel, but once I was in it all I could do was hold that speed. Anytime I applied more pressure to the brakes the wheels would start to lock up. The descent was straight with a 90 degree left and right turn at the bottom so going faster wasn’t an option. After much puckering, I made it around the two corners and the road was perfectly flat (you are riding along the top of a dam). I got half way across the dam and the tire blew completely off the rim. The butyl tube was shredded and the rim was super hot. I think the heat softened the tire bead and increased to pressure in the tire to the point of catastrophic failure.

I don’t think disc brakes would have been any better. Way too much heat for much smaller parts.