The reason new champs (and new skins) are not released at the time of patching is to make it easier to detect bugs, and to avoid too large of a spike of players at once. Think of it this way, if the game starting crashing now, you know for sure Aurelion Sol is not to blame. If the game works perfectly, but then starts crashing as soon as Aurelion Sol is released, Aurelion Sol is most likely the problem.
Also, everyone updating is already hard on the servers. If you had the rush of players that come to play the new champ at the same time as the patch, the servers might crash, or at the very least login times will be insane.