Mostly I think it would work because Covenants are such a major part of the experience that each one deserves its time in the spotlight where all the quests are about building up to a raid that will fix whatever problem is plagueing that area.
For Venthyr there is obviously the class divide and general problem created by Denathrius, we solve it by defeating him in a raid.
For Necrolords there is the infighting, at least partially caused by Kel'thuzad, and I am pretty sure he isnt fully defeated at the end of the Necrolord campaign.
The Night Fae has the Drust, which fits perfectly as a 9.2 "breather" patch in a similar vein to how 8.2 was all about Naga, or 5.2 was all about the Thunder King.
The Kyrians are the most vague, but with Uther damning Arthas in the Afterlives short I imagine a raid would be a perfect capstone to that story.
In short it isnt necessarily about just giving each covenant a raid just because, but rather that they are each important enough that even a player that only picked a single covenant deserves to have a truly epic moment in the expansion dedicated to their choice, and nothing is quite as epic as a raid.