having gone from Linux to HPUX and now partially back to Linux as the OS i make my money supporting, im not sure the learning gap between Linux and Commercial Unix(tm) is big enough that there was ever a real problem training/recruiting Unix(tm) admins.

I think the the real reason why Linux supplanted commercial Unix(tm) lies in the hardware market, around the time when Linux got good enough to compete directly with commercial Unix on stability we also saw the x86_64 systems getting good enough to complete with Power/Sparc/Itanium based systems on most workloads.

And as the Hardware vendors challenging the commercial Unix market with cheaper Linux boxen, were often the same vendors who sold commercial Unix Boxen, the transition was often managed more then fought.