I actually just sold my carbon wheels and downgraded back to aluminium until I buy a disc bike for the long term.
The worst problem is after a few minutes of constant breaking, which you have to do if you're descending a mountain carefully and care for your life, the rim brakes on carbon start to squeal and you lose a lot of braking power. You also used to risk delamination but that isn't that big of an issue on the newer wheels.
Didn't realize what I was missing till I tried a nice set of hydraulic discs, I'll never go back.
I don't agree with the "gap is much smaller than people would expect". Have you tried a decent hydraulic brake? I raced on and off road, was a bike messenger, designed and built (from tubes) a mtb frame, and bike commuted nearly every workday of my life. Disc brakes have significant advantages over any rim brake.
Don't knock them till you try them, I was dubious myself.
One thing I love about my brakes is having a second set of brake levers inline on the top of the bar - I believe the Shimano GX (gravel) series has that now for hydraulic. If I ever have to buy another gravel bike, it will have this feature.
[0] https://www.renehersecycles.com/myth-12-disc-brakes-work-bet...
Myself, I'm not getting rid of my rim brake bikes any time soon, but it's interesting to observe how different people experience cycling.
Roads are far from clean, and if anything the debris is more abrasive than you'll find off road. I'd rather have normal dirt than tire and disc dust you'll find on the road.
Do yourself a favor and try out some decent hydraulic brakes. Now only are they pretty amazing, but amazingly consistent. Even in rain, heat, cold, etc. Also it's MUCH rarer to have to undo a brake, just because your wheel went out of true.
Try to track your use, going downhill and hit a puddle, do you drag your brakes? Why? Wait till they grip again? I had no idea I had such habits.
I've tried hydraulics on friends bikes, definitely nicer, but again not enough to motivate me to switch.
That's how it is with all of these improvements. Someone will always have newer, slicker gear than me, but I keep riding. ;-)
Sure I get it, a stoppie shows that the brakes are not a bottleneck, it's the traction. So sure, sometimes, on some bikes, with some riders, under some conditions you get perfect braking.
However in the real world there's various rim surfaces (aluminum, steel, carbon fiber), with a variation of water, grit, sand, snow, ice to complicate things. Cable have friction and slack. Rims sometimes are not perfectly round/true in both horizontal and vertical directions. Brake pads are rarely perfectly aligned (the alignment changes as the pads wear), rarely nice soft rubber (they glaze over with heat and/or age), and are rarely braking on perfectly clean rims.
So yes, I've done a stoppie with rim brakes, but the average performance of average hydraulic disc brakes greatly exceeds the average performance of rim brakes. Their other complaint about being grabby is highly brand specific, plenty offer great modulation. Not only to discs brake better, modulate better, and tolerate temp changes better better, but they also tend to apply more even pressure. Even small differences in trueness tend to create spots on the wheel that brake less or brake more.
I didn't realize how big the difference was, or the habits I had with rim brakes. Took me awhile to unlearn dragging my brakes when ever I hit a puddle, waiting for them to catch again.
Sure if you have a pro mechanic realign your pads before every ride you'll rarely run into an issue that rim brakes handle well. However if you spend more time riding than maintenance discs will serve you well.
There's a reason even the road racers, the most conservative of the bicyclist, are switching to discs.
Brake pads on rim brakes are a nightmare. They follow a 3-d arc as they approach the rim, which is itself a very complicated 3d surface. Ideally the front edge of the brake pad hits slightly before the rear edge, to avoid squeal... but not too much or it will engage too softly. Even minor wear changes the engagement point. Cables need maintenance, they wear, get kinked, require lubrication, and it's easy to get the tension wrong.
It's all a mess, and even when perfect, and it's dry, not too cool, not to hot, not dusty... they still get worse with every use.
On discs it's more like replace the pads when worn (no adjustments, just remove the pin, pull the old pads, push the cyclinders in, put in the new pads, and put the pin back in). Sure there's occasionally a need for replacing brake fluid and/or bleeding. But generally way less maintenance, and you get like 90% of perfect braking all the time instead of 10-90% depending on temp, wetness, ice, grit, state of tune, etc.
Also MTB's generally accommodate the widest tires, except for fat tire bikes and generally wider tires can be very helpful when commuting for comfort and making things like a random rock or walnut much less exciting when you hit them.
Commuting with rim brakes inevitably destroys the rims, try commuting for 3-5 years then closely inspect your rims, little micro cracks are not uncommon, and will get worse right up until you have a rim failure.
It's also really nice if you do hit something hard enough to knock a wheel out of true, that you still have two working brakes.
That's just not true, there's plenty of braking in the mountain stages. I sincerely doubt that going 90-100Km/h into a hairpin is easier with rim brakes.
Ineos had two accidents this season on descents, first with Moscon in the Giro and then with Thomas in Criterium du Dauphine and even if they might have been just flukes, I'm sure the team is asking themselves questions.
Also, the disc is a consumable on both types. I have worn down rims to the point where I was scared to use the wheels.
The initial setup can be a bit annoying with hydro discs, but it's a tradeoff I'm happy with.
It served me well, but I am still somewhat cautious when descending since then.
So cable-pull disc vs rim brakes is less clear, more susceptible to case by case differences.
However hydraulic discs are always (AFAIK), double sided, and almost always better than even the highest end rim brakes.
Not to mention who wants to destroy their nice rims by dragging brakes on them, leading to relatively short lives as the rim walls get ever thinner, right up till they start cracking. I'd rather replace a disc than a rim after 5-10k miles.