They won't have to train twice as long, because a great portion of those long days are wasted due to fatigue. All these reasons are excuses for sweatshop conditions that wouldn't be tolerated elsewhere.
Have you gone through residency training? While we both consider the conditions inhumane, my wife is absolutely clear on the fact that even at the end of a shift she's still learning a ton. Surgical residents commonly stay beyond their shifts to scrub in and assist or observe especially interesting cases. Based on my conversations with my wife and other residents, you would absolutely need to extend residency to get the same training with fewer hours.