I was one of those 400-hour "whiz kids" that so many speak of, and am now flying as a captain for a 121 carrier. I was concerned about the PIC requirement when it came time for upgrade, but after checking with our company and the FSDO, I found there is a waiver process that is approved for this sort of thing. Basically, it uses the time you fly as SIC in a 121 operation in place of the PIC time requirement. After all, the time you fly as an SIC should be time you are learning and molding yourself into a good captain.
Do not think this problem is only with low-time airline new-hires. This problem can occur as well for those hired with low-time in cargo, corporate or charter operations. Not everyone gets their start as a flight instructor, although I still believe that is one of the most rewarding positions a pilot can have: Teacher and mentor.
Even as a low-time hire myself, I am somewhat concerned about the current trend in airline hiring. It has nothing to do with the quantity of flight time, but rather the quality of flight time of the applicant, as well as their attitude. I also think there is a natural progression in the development of a pilot that directly relates to the type of airplane they fly, i.e. glass cockpit, conventional gauges, piston, turboprop, jet. There is just a certain way us humans can be better trained to conceptualize things, which is highly critical in the way pilots operate. But that is another discussion for another day, and I'll refrain from going down that road at the moment.
Remember, there have been many times in aviation when pilots were hired with little or no experience to fly sophisticated aircraft for that era. It can be done, but it needs to be done correctly. In all honesty, the biggest problem now is the one I alluded to earlier: The quality of the flight time of the applicant. So much time is spent worrying about developing an "airline" pilot that the whole "pilot" part is missed.