While I do believe that a Sandbox Mode would be extremely useful for some people that actually want to take advantage and practice a few things, I don't think their reasoning is really wrong.
There are many things I would love to practice without having to wait for dumb things like "Cooldowns," A very common one, which walls you can jump over on specific champs with their abilities. Or which walls are actually flashable, And then find out the perfect spot to actually use that ability to get across terrain quickly. It'd be great to practice these things.
I think it'd be great to practice a few things.
But I believe that this may create a situation that is current in those Co-op vs AI games.
I have some people I know (and used to know) that played nothing but games that were not Summoners Rift, mostly Co-op vs AI players. Their reasoning, like many others, is because they knew they were preform terribly in a real game, so they decided to keep hiding in Co-op vs AI games and other games modes, hoping to improve in the game in those game modes.
Now I'm not saying there's a problem with this, but no player that actually wants to improve will gain anything from anything that's not a real Summoners Rift game. This being said, I'm sure it can be related back into Sandbox Mode. People may create a way of thinking, that they will suddenly get good at the game just from spending time in Sandbox Mode which, remember, is not a real game of League of Legends. Just like the other game modes, people may just get sucked into this one and never come out, and if they do, they would not have gained as much experience as they could have gained if they played a real match.
People might think by spending a lot of time in Sandbox Mode, that they will improve just because they learned a few more mechanics on a few champions, but being able to apply that to a real game is what is much more important if you want to improve.
The main point of that was, people should not have to rely too much on Sandbox Mode, because it may end up just trapping people inside this new gamemode and no progress would be made.
But this isn't the only problem that may occur
A little quick one, a problem that may come up with Sandbox mode, is that it might be too much fun! I'm sure it has to (or should be) like customs, and you can invite and have people join you. This will just create a bunch of people playing Sandbox Mode in customs, and instead of actually doing what Riot (probably) intended for, has now become a new fun gamemode for everyone to play in, instead of it being a place where you can practice skills. Which leads into less people getting sucked into customs and Co-op vs AI games and also not creating a person's skill progression.
(Please keep in mind I assume that people who play League of Legends actually want to improve, though I know there are a few who purely play for fun, my perspective League of Legends is being a place where people want to get better at the game, and have fun when you know how to play the game)