As a plus, the entire reason that the 90 boost exists is because the primary market for increasing the player base right now is returning players rather than new ones - because other than younger kids that are just getting in to video games, there really aren't many "new" players anymore. If you're interested in MMOs at all, chances are, you've played WoW at some point in the past, so you know what to expect, and you're probably going to be more interested in hopping in to the end game or playing with your friends that still play, rather than soloing your way through 1-4 expansions worth of now-outdated leveling zones that you missed just so you can start playing the "important" part of the game.
And it works. There's at least 4 new players just in my guild that are labeled "<PersonX>'s Friend" in their guild note that I've never met before, and yet are magically level 90. All of those people bought the expansion, so they're going to be back at the start of WoD even if they quit for now after their free month.
THIS part, however, I totally disagree with. Blizzard has kept WoW on top of the market for almost 10 years, and even with somewhat declined numbers due to its age, it still has 5x more subscribers than any other subscription-based game has ever had at its absolute peak. If they want to keep up the same quality in their development, they have to make it worth the effort, and that means keeping their subscribes to a reasonable level, and they know exactly how to do that.It's actually both laughable and pathetic.
Blizzard isn't a charity, it's a business. If you're planning on playing WoD, getting a free 90 now is a good deal, no matter if you're a new player, a returning one, or a current player that was going to stick around until WoD anyway.
You enjoy the product, they make money, and things keep running smoothly for the investors. That's the very definition of good business practice.