In programming we have debugging. You have a program that does X, but with some bugs. You later improve the program by removing the bugs.
Why can't we do this in "real life" as well? You learn how to add multi-digit numbers from right to left. You then later relearn that by going from left to right. You learn to swim with your head above the water, then later learn to keep your head in the water, and turn it every two strokes to get a quick breath.
In fact, I read about this concept of "debugging" bad habits exactly in the context of juggling. Seymour Papert covers this in Mindstorms , p 111. He explains that the most common "bug" that prevents people from performing 3-ball juggling is following one ball with the eyes. Once you are aware of that, you the fix is quite easy: keep your eyes pointed at the apex of the ball's trajectory. In a later chapter he goes on to say that other things can be "debugged" as well; one example is relearning skiing to replace a v-type position to a parallel ski position.
At the British Juggling Convention I taught a workshop for absolute beginners to pass 5 clubs following http://passingpedagogy.com/ . Most of those who'd never picked up a club got on pretty well. Whereas some people who'd passed clubs in a different way were skeptical; you could tell their heart wasn't in it, and then unsurprisingly some of them didn't get it.
I do understand the resistance to going back to basics to fix things though. I can hoop (as in "hula-hoop") fairly well, and know some tricks. But I mostly hoop in one direction (counter-clockwise). If I try to do a trick clockwise, it's frustrating and I don't feel like carrying on, so I tend to give up, or go back to hooping counter-clockwise (fortunately one of my favorite tricks involves reversing the direction of the hoop). This weakness of mine was actually really useful (back when I still hooped with people). If I was showing someone else a trick, I could try it clockwise, which was an excellent reminder of how hard the trick really is, and to understand how/where it goes wrong.
One thing that your idea prompts is thins: as I have gotten better at learning things I have gotten better at just adopting as-close-to-perfect form as I can from the get go.
When I learned to play guitar at age 20, I had horrible form, and I did go through a period of unlearning habits (after a period of trying to have a "style" LOL).
When I learned to play pedal steel guitar in my late 30s, I was careful to start with good habits from the get-go. Same with snow skiing, banjo, and yoga. :D
I dunno how I'd approach this lesson when dealing with younger folks... it was a painful process, but learning that starting with good habits/ form makes things so much faster and easier is maybe just a thing people have to experience on their own.
Tom Brady, who many people consider the greatest quarterback in the history of American football, still has a throwing coach (Tom House ), and he's still debugging his throwing motion. After 20+ years of throwing in a professional league.
So, for sure, unlearning habits is difficult, but learning only proper form from the start is probably an exceedingly rare exception. I think for most people the process of learning will involve learning incorrect form first, and attempting to fix this later.
My pole dance teacher once pointed out how different my style of training was: most students would try a difficult new move a few times, then go back to practicing stuff they were ultra-confident at for a while, while I'd be more likely to keep on trying the new move with a bunch of different little variations, and to pay a lot of attention to her when she'd come over and point out things I was doing wrong, especially if it was a wrong thing that would make me more likely to hurt myself!
I kinda feel like I can do this because I remember how I improved my art by the long, painful process of analyzing what I was doing wrong. And also because I have much less ego invested in the new thing - I already have a thing I can do the heck out of, I don't give a shit if I look like a bumbling beginner when that is what I am.
"How to learn" is a skillset, which you have to learn along with everything else you learn in the first two or three decades of your life. Once you have it down it's a lot easier to learn stuff if you're willing to put the energy into doing it right.
Or is that just one of possible approaches? Or does pretty much everyone draw from boxes?
You start by drawing boxes. And balls, and tubes, and eggs, and cones, and prisms, and a bunch of other shapes that are simple enough to describe in a few brief lines of code. Get good at them, learn to draw them from a lot of angles, learn how to think about them as three-dimensional shapes and how light plays across them.
Then start laying out rough, crude versions of things using these primitive shapes. What you use for a particular thing depends on what you're drawing and what suits your approach. Cars are big boxy things, maybe big wedge things if they're really areodynamic. People are mostly collections of long tubes, though some parts can get very boxy, eggs are helpful for some ways of constructing skulls too. A lot of people go through a phase where they like to draw stick figures with balls at the joints, I've never really been a fan of that and find it tends to result in stiff figures, but some people love it. Sausage people, box people, ball-and-stick people, there's a lot of ways to approach this and a pro will have played with them all and found out which one works best for them most of the time, and which ones work best for them in situations where their favorite way breaks down.
You work out a pose this way, as a bunch of sticks and balls and boxes and whatnot, then you have a solid framework to work on top of and sort of "carve" into a more realistic shape by applying your knowledge of anatomy. Which is a thing that takes multiple years of study to acquire, human bodies are complex things!
(Boxes are especially useful because there are some simple tricks you can use to make it easy to take a flat view of something and project it into perspective - if you draw an X from corner to corner on the face of a box in perspective, then you can draw a line that goes through the center of that X and lines up with the same vanishing points the sides are on to divide the face in two in perspective, then use a grid built up that way to transfer a head-on drawing into perspective and work from there.)
Eventually, as you progress as an artist, you can do more and more of this in your head. Most of the time I just lay down some really sloppy, loose shapes to plan out a pose, with a lot of parts going pretty quickly to a recognizable caricature of that body part that I can quickly turn into something good-looking when I come back and throw down some loose solid color shapes that I quickly refine into something with an appearance of anatomy, then come back later and add some shadows/highlight to really bring out the forms. I'll put out a little bit of cubes/balls/eggs/cylinders/etc when I really need to think about a weird angle, but every time I do this a little of this lingers in my head for next time, and drawing that angle again becomes something I can kind of... pull out of cache, so to speak, because I remember all the thinking I had to do on the page last time.
Loomis' books are super solid and have a lot to teach you. Bridgman is some super useful reference for anatomy too. But the teaching that really helped me the most was a life drawing for animation class whose instructor was working out of the Vilppu drawing manual, that stuff is amazing and will help to keep you thinking about how to instill a sense of life into all your work from the ground up.
The same stuff applies to simpler cartoon characters, too. You just use different proportions and don't spend as much time trying to nail down anatomy that isn't absolutely necessary to the story the drawing is telling.
Or, do they? If a program is built with a bad architecture, but "works" for all the inputs seen so far, it's much hard to fix than if it were built with good patterns from the start, even if it has some mistakes that need to be fixed.