I used to think this was the way it was; in fact, my career path resembles the ideal situation alluded to by the OP: preloader->driver helper->skilled driver helper (i.e. knows thoroughly how to use DIAD)->learned stick (trained by sups on job)->driver. However, being on these boards (not to mention talking to the majority of drivers in my hub) I see my path was atypical, and now I find myself stuck on thr ladder. Next up for me is feeders, but the demand isn't the greatest in my local, although jobs are available; I think i could get picked in the next two or three years. However, two large feeder operations (Willow Grove/Horsham, Pa and Lawnside, NJ) are close by, and desperately need feeder drivers; however, we are different locals and have no access to those jobs, short of quitting UPS (losing our senioroty) and hiring on at Willow Grove as a new hire.
I believe allowing transfers between buildings between regions can help. In my hub (and others, like the aforementioned Willow Grove), there may be more opportunities for advancement. Even now we have been asked to get referrals for utility drivers as the dovetailed list has been exhausted. I don't believe I put this as one of my desires to see added to the contract, but I intend to for the next one.