Your question is worded poorly. I have accepted that their are disc brakes on tri bikes. Not accepting it would be like me saying that the world is flat.
Now, what would it take for me to actually buy one of these things? Well, I'd have to want a certain bike that just happened to have disc brakes. And not only would I have to want it, I'd have to want it enough to deal with selling all of my non-disc wheels and buying the new more expensive disc versions.
I don't see that happening anytime in the next 5 years. But that may be just because all of the new disc tri bikes out there right now are uninspiring.