Children have lots of places that they have no power to control their lives - it does not teach them to be powerless. That's a modern fallacy. Moreover, there are lots of areas, including areas wherein a child's desire is governed and ran through their guardian's sense of wisdom. Again, this does not ruin children.
Another way of looking at this list, is to go ask someone that you think is good at Aikido - your sensei, for example: Ask them if they have always loved Aikido training, if they never went against their whims and fancy in order to continue training, if everything they ever did and/or accomplished in their Aikiido was done so only at their full and complete volition, etc. See what they say. Or, if you know a master of another trade/art, ask them.
Or, another way, if ask yourself how good little Jonny is going to be when they only commit two hours a week to training, cancel some of those classes during soccer season, decided they like baseball better, and video games, see no reason to train in Aikido outside of techniques on the mat, and then quit.
Sure, you might be able to say he has self-discipline, he's self-empowered, etc., but you won't be saying he's skilled at Aikido. Along the same lines, no matter how many Jonnies we know, one cannot by extension say that any child that sees his/her way through the rigors of sincere training, unlike Jonny, can in no way be self-disciplined, self-empowered, etc.
I can see your point in that you can't be good at Aikido if you quit. In fact this is one of the things that kept my oldest boy in Aikido when he wanted to quit. He was feeling too much pressure. I was giving heim to many critiques after class. He expressed this by saying he wanted to quti Aikido. We discussed it and he did want to become good at Aikido. What he needed was more room to fail. He understood that he couldn't do Aikido if he quit. However, quiting was an option for him. If he did not want to become good at Aikido he had the option to quit. He had to know the consequence of that descision but it was his descision. You are right that if he quit he would suck at Aikido.
As a parent, I think it is important to identify when it's right to quit. It can't be as you say on a whim. You need to understand why they want to quit. the reason can be real and reasonable. You may need to persuade, encourage and insist that that they continue if the reason for quitting is not one you find reasonable. But for ne, it is a dialog. I need to listen and understand them. I need to know why they want to quit.
As for asking Sensei those questions. We both know the answer. We both know that they made sacrifices and sweated more than they wanted. We also know that in all likelihood they always had the option to walk away. They always chose to make the sacrifices and loss of personal control. They chose to place their fates into anothers hands. They always had the option to quit. They did not but they could have.
As for time on the mat. We are at the dojo Monday, Tuesday, Wednesday or Thursday and Saturday when it is not Soccer season. We drop the Saturday session which is a little kids class that his brother goes to. If anything he's at the dojo to much. I will agree that two hours is not enough to get better especially if that is the only time that Aikdio is practiced. This week we are down to only two hours because his little brother blew chunks in the dojo parking lot before class but things happen. If we ever fall to a two hours being our regular session we would be discussing quitting something but that something will be his choice.
To be honest, I don't think we are that far from agreement. I just leave the option of quitting on the table. Quitting means they lose Aikido. They lose what Aikdio brings. However that is their choice. I do not view this as a detriment to their training. I have seen to many kids forced to do things their parents think they should and get almost nothing out of it. I've coached to many kids in soccer that are a deterament on the field and a problem in practice to think that forcing them into what they don't want to do is good for them or thier team/dojo mates. To be honest they suck, too.
If this is a modern view so be it. It is a view I have adopted from seeing it in practice. If the sole motivation comes from outside, they will suck and worse they impact those that are are their of their own choice.