Sure, but how does having him a 10.3 villain change that dynamic any more than TBC having Kil'jaeden entering through the Sunwell?
It didn't make all of TBC center around Kil'jaeden. His story was just a footnote in the rest of the TBC storyline, even though he was a cosmic level threat.
Why are you regarding Murozond any differently here just because I made an example of him being a Raid villain? If Blizzard simply wants to focus on Nozdormu's story as a part of 10.3, then that's what it is. It doesn't suddenly root the rest of Dragonflight as a 'Cosmic' expansion. We're just talking about resolving a Bronze Dragonflight storyline in one of the major raids; with this example being the final raid before our next major adventure.
I mean, what do you consider 'the main story' of TBC, and how do you consider Kil'Jaeden's involvement in that story overall as a cosmic level threat? If you're talking about taking things back to how things were in Vanilla and TBC, then you have to clarify what you actually think TBC was all about.