MonteCristo seems to be intentionally misunderstanding...
Here's the deal, Monte. You are right about your positive points. It WOULD help pro players improve to have sandbox mode, and I actually kind of agree that there should be one. Consider me on your side.
What makes me irate though, is when people fail to make any concerted attempt to understand the argument being employed. Riot ALSO agrees that players would improve with this mode. What they are saying is that they think it would contribute to community toxicity to have this mode.
Think about this: any time you fail a flash over a wall, your teammates can tell you that you aren't good enough to be playing ranked yet. It becomes a requirement that players have to practice in the sandbox just to avoid being complained at by other players. THIS is the higher barrier of entry Riot is talking about.
You can disagree about whether or not that could happen. You can disagree about whether or not that's a good argument. But please, don't act like you don't understand the concern. It's a real concern. Riot is trying not to alienate players, and that's a good motive. Let's not demonize them just because they disagree with you, please.