I agree on the stepped progression, in terms of learning more skill, then being molded into responsible professionals.
I'm not so sure that the city is the best place for the true rookie. It's certainly not the safest. All of the concerns mentioned are multiplied in terms of quantity. More hazards, more often. The road has different challenges that also need experience to conquer. Probably the worst place to learn, while the most common, would be OTR (irregular route), IMHO.
LTL probably provides the best mix, if introduced properly, methodically, etc. 1st Yard, then swaps, short line haul, etc. all provide known challenges. The key element is caring enough to introduce new challenges, as appropriate, starting with the foundation. Too many get thrown to the wolves, placing everyone at risk.
I'm not so sure that starting on the dock contributes anything to becoming a safe and responsible (defensive) driver. It does provide perspective on cause and effect on the cargo, which is valuable knowledge, but does dock experience translate one directly into a "safer" driver?