Well it means they had shitton of time when they weren't developing any more content for MOP so that's supposedly the time they were supposed to develop more content for WOD... which didn't happen. Also it was 1 year not 2 but w/e.
WOD had a massive sub drop off because of 1) technical mega fail where the game was unplayable for a week, lags, dcs, garrisons not loading, and so forth, 2) 6.1 being a patch with nothing in it except belf models (ok there were some skimpy amounts of quests but not much) 3) missing at least 1 major patch and 1-2 minor patches to be a full fledged expansion.
If there was a proper mid xpac patch with Shattrath hub + raid and Farahlon, but no, we got half an expac for a price of one.
You could say it was a self fulfilling prophecy, people left due to no content, so even less content was developed?
There's also the problem of dealing with technical debt: phasing / garrison tech and new models were coming behind schedule, both with bugs, it was a bumpy road for them. Basically the same thing that crippled Anthem, EA told Bioware to develop on an engine that was lacking so they spent more time fighting with the tech than actually developing the game, so it came out poor and unfinished and content starved.