I think people mistake the aftermath of abyssea for abyssea itself. From my experience abyssea and the level cap increase brought a lot of old players back to the game and people started quitting AFTER heroes came out. I think it was a combination of 9 months of instant gratification and SE not having anything ready to go after abyssea cause they assumed 11 would be a ghost town. Which is why we got low production value time sink voidwatch thrown together to keep us busy while they used all our dev money to rebuild 14. That's when all those people who came back started quitting as far as I could tell.

I've said this before but all you need for proof that abyssea was both a financial and critical success is the fact that Yoshi is the man they went to when 14 needed saving. They wouldn't put someone who had just directed a flop in charge of saving an already sinking ship.