Hmm, agree and disagree.
I agree in the sense that, that is how I typically played. I learned in bots, then tried in normals. When I get to 30 I'll use them in ranked and will have a reasonably sturdy pool of Champions. I think it's the appropriate way to learn a champion without hindering other players.
On the other hand, there are a lot of negatives. First, how many games? What if I got really good with a champion after just 1 game because I main that role and match up against that champion all the time? Will toys cause people to just afk in bot Games making it harder for new players trying to learn there (make it that you need to win?). What about people that played that champion on another account? And most importantly, what if I'm playing Draft pick (normal or ranked) and it's just a freak occurrence that I don't get my preferred role AND the champion I use in the off role in stuck in was picked/banned? I might have 3 champions in an off role and see 1 picked and 2 banned? Thus I'm forced to pick a champion that isn't supposed to play that role even though I might do okay/well with a champion made for that role who I haven't played yet.
I just see a lot of potential problems. I agree that players should choose to practice in bots before going pvp. Perhaps the game should keep track of how often you play a champion for the first time in bots or in pvp and adjust matchmaking in normals to account for it. I'm against adjusting in ranked though because people would purposely not try in bots to be matched easier and win more. It's really hard to see a good solution.
Hopefully you or someone can see my concerns and come up with a better way.