Riot's reasoning behind not working on a sandbox is just stupid.

"We want to make sure we’re clear: playing games of League of Legends should be the unequivocal best way for a player to improve. While there are very real skills one can develop in a hyperbolic time chamber, we never want that to be an expectation added onto an already high barrier to entry." - Riot

This literally makes no sense in other situation so it is crazy to think that League is the only situation where this holds true. When playing basketball for the first time the best thing to do isn't play 5v5 games constantly and hope to get good at everything. If you need to work on shooting, it is best to work on shooting in a situation that amplifies that aspect of the game, like shooting constantly on a hoop to get the most practice. Transferred to League this would be saying that if you want to test out a late game build, it is best to keep playing games hoping to get to your full late game build to see its power, rather than setting up a situation where you can test that build quickly and make modifications along the way at the expense of no one else. Saying that this would become an expectation to playing Normal/Ranked queues is not the case at all, if someone wants to jump into online play, then they still have all the right, but if I want to test out late game AP Zed, I feel like it would be much better to try in a sandbox mode alone rather than at the expense of my team having to deal with a potentially (or in this case certainly) troll build.

"On an individual level, we know this isn’t always true – some just want a space to practice flashing over walls without having to wait at least 3.6 minutes in between – but when that benefit is weighed against the risk of Sandbox mode ‘grinding’ becoming an expectation, we just can’t accept the tradeoff. We never want to see a day when a player wants to improve at League and their first obligation is to hop into a Sandbox. We do want to support your ability to grow in mastery, and there may be other avenues to do so, but not this." - Riot

So rather than helping someone who wants to work on a particular skill in the game, you would rather there be no sandbox mode in order to defend against players going into a sandbox mode to improve on a particular skill in the game? To me that doesn't make all that much sense. A game that first comes to my mind is Halo which has a Custom Game option much like League Custom Games, but also has the Forge Mode in which you can instantly spawn Items and quickly move across the map in order to test certain jumps and and weapons while also being able to create some pretty cool game modes that are fun after being tired of playing a long stretch on Online PvP games. Halo is a game that has much less practical use for a sandbox type mode than LoL and They still have one.

If players want to grind in a sandbox mode to work on a particular skill, then why not give them the opportunity? If others don't want to use that opportunity if it is available that is completely up to them but why punish those who would use it (who make up a massive portion of the playerbase). A lot of Pro players and casual players alike are upset about the fact that Riot has said they don't want to offer the opportunity for players to work on particular skills in a mode like this and it need to be looked at again.

Tl;Dr Riot has said a sandbox mode is not right for League because they don't want to risk "grinding" in a sandbox mode becoming an expectation to playing online with others. This makes no sense in any other type of game (Playing 5v5 games of Basketball without being able to practice dribbling or shooting first) and is just a lame excuse.

P.S. For anyone who thinks bot games are available for this type of thing, bot games will teach more bad habits than good habits.