That's true, but you also don't need (or usually want) to unlock every single champion. Most players have at most 20 champs they're really good with and will typically play. There's also the new mode they recently added, ARAM, that will randomly assign you a champion from the pool of all champions. Playing ARAM gives you the opportunity to try out other champions for free, and if you decide you like one of them you can spend your IP to unlock it.