Remind me how old Moonshine is again?
Once muscles have been made, they are never going to go away.
So a horse that has been allowed to travel with their head in the sky, back hollow, and no collection at all is going to develop a big strong muscle on the UNDERSIDE of the neck.
Can you re-teach them how to travel? Sure. But you can't "remove" muscle that has already grown.
Think of a this way, maybe someone who is a serious body builder. Think Arnold Schwarzenegger. He's never going to be a skinny guy. Not going to happen.
My Dexter is inverted. We bought him at age 10 (I think). So he had 10 years of being ridden incorrectly. He can and will travel collected, softly, etc, when I ask him. But his natural response is based on the muscle structure he has. To some degree, yes, it also depends on what conformation a horse has, as that will influence things (such as how they choose to naturally run around in the pasture). But it also depends on how they were allowed to travel their whole lives.
Our training, and how we train them to move their body, has a life long effect!!
Just like a human who walks around with a limp. You're going to develop certain muscle, and underdevelop others, for not traveling correctly.