This is about the paid 90 boost. There were rumors late last year about massive ban-waves incoming on blizz devs twitter feeds. Most botters are to lazy to do things in game and also buy the game when it goes to special pricing. What happens is that once one account is banned under a bnet account you can't risk ANY inappropriate activity(for the most part :/) on that account(72 hour bans for botting included). After the ban-wave a month-ish ago there were a few comments from blizz devs that indicated a decent percent of the player-base (literally saying that they were shocked and saddened at the numbers) had been banned. So you have a large part of your player-base with extra keys on a bnet account but to lazy to pay the 15 bucks to switch back to the account now that the RaF part is dead. Tbh reading that back sounds like a bit of a stretch but considering their comments on the numbers banned and the fact that the boost is to current cap and separately purchasable and being released so soon after the ban-wave and sososo far ahead of when WoD will be dropping that it makes a pretty compelling argument for why they were willing to offer it.
On a separate note I have no doubt that the general idea behind WoD being to bring back the best of the past with the conveniences of today could bring back quite a few old players in with the right incentive(the boost/model revamp) is important. Being offered now though is too much to ignore. ZOMG last second extra thought. They are offering this right before the next pvp season(most of the botters were abusing an interrupt feature and they weren't originally going to have another season) and there is the catch up mechanic for pvp.
pre edit: sorry i don't have the sources for all the stuff referenced but if you normally read the most recent posts you will know what i'm referring to. Finding it all would take a while but from the time i spend on anything related to hacking/botting(ie i lurk a lot on "those" sites that i can't directly name) out of pure curiosity has given me a decent insiders perspective on what these players are doing.