gsa103

The real issue is the service motos, which only carry wheels. They only have a finite number of wheels, and rim wheels are very standard. With disc you have to make sure it has the right thru-axle, disc diameter, and even then the disc to caliper distance can be off. There are some rules about the support motos carrying wheels to match the top GC, but it may only be the top 1 or 2. In the early mountains, the main GC contenders are usually several places down, but only 10s back.
On the narrow mountain roads, the cars get trapped behind tail of the main group, so only the motos will be up with the top 5. Getting a wheel from a moto is the difference between losing a minute and loosing 5 minutes.

If you're a TdF contending team, you're talking about risking the entire GC on not getting a flat in the mountains. That's a big risk, with basically no benefit. If you have the option of rim brakes, teams will continue to use them.
Next year will be an interesting test. Jumbo-Visma is switching to Scott and the Addict and Foil are both disc only. So Roglic will be discs. Ineos will still likely be on rim brakes, as will UAE.