rsbob 

Mountain bikes when they first appeared had rim brakes or even V brakes. Any mid-level MTB now comes exclusively with discs. It’s what the consumer wanted, especially when fording creeks and going through wet where stopping power is important. Sure there are still some low-end cheapos out there with rim brakes but you won’t find them on a bike of any dedicated bloke.

I believe that the same evolution will take place with road bikes. If you ask a pro which bike would they prefer to descend a long winding rain soaked pass, I would venture to guess that a high percentage would go with discs for their mere stopping power - and teams might prefer them to keep their riders a bit safer, but that’s just conjecture. Someone else posted that it would be cheaper for mfgs to just create one style frame - and economics being what they are, that may end up being the overriding reason. That’s just my opinion, I could be wrong.