There's a vocal complaint, EVERY expansion, and there are always more complaints than there are compliments. In actuality I would say the reason "people disliked cata the most", is because it was the most recent and the people were the most drained of WoW. I saw nothing greatly wrong with Cata compared to others, and enjoyed very many aspects of it, however I did quit in it. So Your "loss of subscription" just doesn't really cut it, because I imagine there were very many others like me, and I imagine the sub number will fall just as much in MoP for this reason.
I quit because I have been playing since 2006, and have burned the game out - alongside the fact I can't dedicated the time I would need to to enjoy it.
My friends quit because they don't want to pay sub fees for a game they don't play as much when games like GW2 are sub free and don't require certain commitments to be made to enjoy fully.
All of us still consider WoW to be a fantastic MMO with the most deep and rich lore including in Cataclysm, with Firelands being our favourite patch. All of us are still quit - we didn't come back for MoP but we were not driven away by Cata.
I disagree that rift was a better game, I tried it on trial - there were some good aspects to it but it was very similar to WoW without the lore I had grown to love through Warcraft and years of playing the game. I also disagree there was less room for improvement with Rift, and consider that to be entirely subjective.