IMO, disc brakes on a tri bike seem impractical. To begin with...I’ve read that disc brake systems can weigh as much as a pound more than traditional caliper systems. Granted, the stopping ability of disc brake is better...to a point...mostly in wet conditions. And, if you’re riding includes big mountainous descents that require lots of braking they can be a big advantage over rim brakes. But how many triathlons include those conditions? All the tris I’ve ever done were on moderate elevation courses, at most, where rim brakes provided more than enough stopping ability. WRT ‘wet conditions’...sure, I’ve done a few triathlons when it’s raining. But most of the time race management would postpone the start until it stopped raining (I guess because of the concern of lightening and swimming). Roads would be wet but not so much to impact rim brakes. I’ve always found rim brakes sufficient for those conditions. And, generally triathlons don’t have major hill descents on the cycling course that would necessitate disc brakes. In other words...triathlons (the bike course) are generally conducted on courses and conditions where the rider would be able to “lock up” properly adjusted rim brakes if necessary. But...the thing I’ve always thought about in the comparison between the two systems is that once you get to the point of having to lock up the wheel...your stopping ability is transferred from the brakes to the few square inches of tire-ground interface. At that point it doesn’t matter if you have rim or disc brakes.
Dan