Sorry if we come across as hostile
But we're not wrong here, you are, Riot.
There are a million reasons to create a sandbox mode, and every single one you've listed to not have it except for those involving development time or resources can be refuted without more than 5 seconds of thought.
Every reason you've given us to not have sandbox mode is complete and utter bullshit and that's a fact. But hey, it's your game. Please, feel free to alienate and ignore your playerbase. I'm sure that will go over well. Just look at Blizzard right now. WoW's down 4.4 million subscribers (nearly half) due to their blatant disregard for all feedback and player suggestions, and continuing to plow on ahead with their terrible ideas and design philosophy.