JDI, a horse can learn by itself to carry himself properly without us interfering. Yes for a long time they might carry themselves wrong....but there will ALWAYS be that time when the take off with themselves properly carried and it will hit them "OMG it's easier for me to carry myself this way!"
Personally, I'd rather have my horse be able to realize it's easier for him to carry himself and have him be able to do it himself, them having me to constantly remind him.
No, they do not ALWAYS go "OMG it's easier to carry myself this way" because it's not. Generally it's really hard for them to carry themselves properly because their muscles are not trained to do it. It will be hard and they will use new muscles and it will hurt for the first little while because it's a new sensation.
Just like for humans - like I said, 99% of us do NOT carry ourselves properly! And if you try to correct it, it's going to be hard work!! Believe me, I've been there! I had a physiotherapist work with me for months to get me to walk "properly" and it was hard. (Now it's easy... but it took a lot of muscle training.)
I would love to meet a horse that was all "omg, I'm going to carry myself right without being taught" because that is very rare.
This is why you have to school them to carry themselves properly. And yes, I agree, a well-schooled horse should carry itself properly without you having to do anything, but to get to that stage takes a LONG time. But once the horse is well-schooled to carry itself properly, you should be able to toss away the reins and have the horse maintain balance and a frame, and impulsion from the hindquarter.