You can't argue that working on the Wheel of Time has increased his recognition, which increased his sales, which earned him a new contract and boatloads of money. Lets also not forget that Way of of Kings probably would've been a smaller book with less extravagant maps, if not for the influence he gained from working on AMoL.

So I understand that Sanderson had already published 3 going on 4 books by the time he started working on Wheel of Time, but he wouldn't be where he is now without it.

So despite making a decent living off of writing before AMoL, I think you could still make a good argument that the WoT series launched his career.