And how are you going to do this without an enemy present? If you have an enemy champion AI then you're pretty much right square back to Co-Op vs AI.
No, because Co-op vs AI does not have 100% cooldown reduction. I'm pretty sure you can insert dummy champions in training mode.
How are you going to be able to do it effectively in a real game if you're used to it having no consequence?
If your goal is to learn how to Kick-Flash on Lee Sin, then you should not be even focused on the consequences of a real game. It's called getting the basics down first, that is, the mechanical ability to even perform a Kick-Flash. Once it becomes muscle memory, you can then focus on one less thing in real games (screwing up the combo) and focus on the consequences.
Practicing dribbling is useless, because normally someone would be there playing defense! Better go practice dribbling in real games first. -Said no one ever.
How are you going to practice learning when you should and shouldn't channel without some form of enemy present?
No one is going to practice "when to use Katarina ult" in training mode. That's something you practice in real games. One might practice in which order to use Katarina spells for muscle memory though.
How are you going to learn how to land skillshots without an enemy there to give some form of dodging?
Again, this is not what training mode is for. Training mode is for learning champion specific mechanics.
How are you going to learn how to use your abilities to help other players without other players?
Again, not what training mode is for.
I'm just saying, it's fairly easy to learn playing against bots, I know several people who did just fine that way- but they STILL kinda fell apart when playing against actual people.
Learning champion specific mechanics does not mean you've learned all there is to the game. Champion mechanics is just one aspect of the game out of many that can be improved upon: minion wave control, matchup knowledge, ward control, map awareness, etc. etc. If you're playing against bots, of course you will not play well versus real people.
I don't expect having an even more casual learning mode is going to make that even less of a problem.
Well first you've completely misunderstood what training mode was for. Second, Practice mode is not even for casual people. Casual people will not care to learn to Kick-Flash in a desired direction on Lee Sin, they'll be content with just Flash-Kick because they don't need to execute their combos faster at unranked or lower elos. Casual people don't need to learn how to do the fast combo on Riven, they'll just hit spam QQQ or weave autoattacks in between Q's at a much slower pace. Casuals won't need to do one shot combos, such as the shy combo:
https://www.youtube.com/watch?v=fIyuZCoSkE4
Ultimately, Riot kinda has a point, the best way to learn the game is to play it.
No, every high elo player recognizes the value of training mode. Riot did not have a point, which is why they admitted that they were wrong and they're finally giving us a training mode.