It is what anyone needs to live a fulfilling and happy life.
"Discipline" in the sense of submission to authority you don't need. In the sense of being able to follow rules in a team, yes.
"Self-discipline" in the sense of internalizing what authorities have told and identifying oneself with the authorities, no. In the sense of being able to follow own decisions, overcoming temporary whims, yes.
They have a little of it from a very early age - but a very little of it. It increases with age, and eventually self-discipline is able to take over from the discipline imposed by a parent or teacher.
The ability or the will to fight - that means, to overcome some obstacles to reach the own aims - is more or less strong already at a young age, it is more character trait than education.
And, no, I don't believe in such a takeover. There are two variants: The child accept submission to others - then it will be submissive all the time, follow others instead of making own decisions. Or it doesn't - then the "takeover" is simply the point where it becomes strong enough that parents can no longer force it to do what one does not like.
But, of course, this does not mean that one will do something completely different. If the child is sage enough at this time, it may be able to recognize that what the parents have tried to force him to do was in his interest. If not, shit happens.
Yes, all already grown up.
However, abandoning it because it is too hard, or does not make a child instantly happy, has bad effects long term.
This is a different question, but you should not think that education has much effect at all. 50% is character, 50% is environment in the widest sense - with parents, siblings, teachers, classmates, peer-group, TV, books as influences, and most of the influence being in form of being a role model. Thus, intentional education gives something below the 5% level on influence.
Or remember anything from before you were an adult? I'm a 39 year old, successful engineer, but I was not self motivated as a child. I could not possibly have gotten where I am without parents and teachers forcing me to learn against my will.
This is what I doubt. What means not self-motivated? You wanted to lay all the time in your bed doing nothing? Or you simply didn't like to do the things which you today believe have been useful to you?
If you were simply interested in other things, I'm sure if you would have done these other things, the results would have been fine. The amount of things you have learned is, in fact, quite irrelevant, given that those who learn with self-interest learn much faster much more. You would have learned different things, much more of them, and the side effects would have given you enough in the other domains too.
I was quite similar to what I'm today already as a child. I liked math quite early, also played chess, at the top level of my home town. But after some time disliked it. My father had similar thoughts that it would be better if I would continue, and learn to overcome these whims. The only result was that I learned how to overcome a slightly different particular problem, namely the will of my father. After around half a year I succeeded. With mountain hiking and alpine skiing as the sport disciplines I liked I had sufficient opportunity to learn how to overcome difficulties.
School was nothing but a total loss of time. It would have been much better if the teachers would have allowed me to read books of my own choice during this time. Given that I had interest only in math, not at all in physics, I have essentially not really learned anything about physics at school. Sufficient for exams, ok, but nothing beyond. My interest in physics came later, completely independent of the school - so that I started learning physics with relativity.