Which is a really odd assessment to make. There is no real "winning" in World of Warcraft, unless you want to count sweeping a PvP season, or maybe being the first to complete all heroic modes of every raid in the current expansion. The thought of "winning the game" is an arbitrary, muddy label that doesn't actually apply to anything in Warcraft -- it's not a game designed with a finite endpoint or a finish line you can cross.
Let's be clear, here -- level 90 boosts may not be for you. But that doesn't mean they shouldn't be available, and it definitely doesn't mean they're going to kill the game.
Bashiok wrote a blue post on this subject that addresses the situation pretty well.
Here's the deal with level 90 boosts. There are several situations in which this feature will be used.
Current players Players that are currently playing the game may have an alt they'd like to get to level 90 quickly, so they'll choose it for the boost. Or maybe they decide they want to see the other faction's content when Warlords comes out, but they don't really want to level an alt all the way through 1-90 on the other side, so they choose to boost a character for that purpose. These are also the people most likely to pay an additional fee for more level 90 boosting -- if they've got a stable of alts and they've only managed to level 8 out of the 10 characters, maybe they want to pay to boost the other two.
Returning players Players returning to the game after a long hiatus might want to catch up to their friends. After all, by this point in the expansion everyone is out there doing endgame content, and have been for quite some time -- why wouldn't a returning player want to get in and play where all the people are at? Returning players might also want to purchase additional boosts for the same reason -- they have a stable of alts that they'd like to catch up to current content.
New players Players that are completely new to the game and have never before played it. This is where it gets a little nebulous and tricky. Imagine, if you will, going into a store and seeing a box for a really cool-looking game featuring a bunch of menacing orcs on the cover and a lot of really cool features listed on the back. Maybe you played the Warcraft RTS games once upon a time and you recognize the characters on the box -- Grom, Khadgar, Gul'dan, Ner'zhul -- and the thought of them being featured in an MMO is pretty cool. So you fork out the money to purchase, bring the box home, install it and hit play.
Not really. Not at this stage of the game. Everyone else is already at level 90 and playing through the new stuff -- why shouldn't a new guy have the opportunity to play through it at the same time as everyone else? Why should a new player have to wait and slog through that content months after everyone else has already experienced it? That guy could be a new raider, he could be a new arena partner, he could just be a new friend -- but if he burns out on the idea that he's never really going to "catch up" to everyone currently playing at max level, we'll never know.
That new player is probably the least likely player in the world to purchase an additional level 90 boost. Why? Because now he has one character that skipped everything and gets to play with everyone else under the sun. So why would he pay to skip another through everything, when he's got the chance to go back and play through it all at his leisure and see what he's missed? If he wants to, so be it -- it's his money, after all.
But this is an opportunity for those of us that have been playing for so very long. It's an opportunity to embrace the new, grab these new guys, show them what's what, and get down to the very important business of killing dragons on the internet. We've got a game we already love. Why would we ever refuse to share it?