That packages it up nicely, thanks.
I've got a couple other thoughts on how the books I referenced apply to skiing.
First, it's really difficult to get enough mileage skiing to become an expert. I've probably averaged about 30 days/year for 30 years. At around 6 hours/day of skiing that puts me around 5,400 hours. That's barely half of the 10,000 hours the studies show it takes to become an expert at something, bummer. Since skiing is seasonal and recreational for most of us, it's really hard to get in the time.
Second, most folks don't challenge themselves enough when skiing for it to be considered "deliberate practice." Deliberate practice is by definition difficult and probably wouldn't be considered fun by most. I personally think continually skiing terrain and conditions well beyond your comfort zone is the best practice for becoming an expert skier.
Exactly why it's hard. IMO, it's the deliberate practice which is the biggie, though. Since, as you outlined in your first point, most skiers just don't have the opportunity (whether financial, other obligations, or other) to get a lot of mileage, understandably the last thing many want or have the time or money for is deliberate practice with a trained and knowledgeble professional...why not just go have fun?