I feel like co-op v AI does the job sufficiently well... The average new player can barely manage against easy bots. Once they can hold their own in intermediate bots, they should be ready for PvP. The matchmaking system also should only be queuing them with other fairly new accounts and inexperienced players. Excluding the occasional smurf, it is essentially that queue. There may be a few extra champions (e.g. a 1350IP champion could have been on rotation the previous week, a new player tried and enjoyed it, and then bought it) but that's a good thing isn't it?
I'm not saying I'm against it. It literally wouldn't hurt anyone anyways. I just don't see how it's different than the current environment. If anything, due to not allowing copies of champions, it would be worse. IIRC, pretty much one of the only things I disliked as a new player was that whenever a really cool champion came on free week everyone wanted to try that champion and it took a lot of tries to select it first. Some weeks I even never got to try the champion I was hoping to because an ally always managed to select them first (or the only games I selected it first, an ally dodged because they didn't get it).