Holy was outperforming Discipline in all of ICC (exception: Arthas, but not from a numbers standpoint).
People still favored Discipline because its absorbs made it a feel good spec on meters, and hey what raid doesn't like an extra cushion? It may or may not actually be necessary, and from a numerical standpoint still be theoretically behind in raw output, and the spec will still be the first spec Priests go to when pushing progression.
Will nerfs push Disc down on meters a bit? Yup. Will they push a fair number of Priests towards Holy, whether it's because they feel they can push higher numbers, or because hey maybe they want a break from Discupline and want to try something new? Of course. Will these nerfs alone break the consensus that the community at large has come to, that even when numerically inferior, absorbs > healing 99.8% of the time, exception made for gimmick encounters? Nope.