With regard to the part about people being difficult to replace - I think software and software management will be one of the last things ever to be automated. As such at present it is one of the last bastions of truly hand built design. Anyone who is good in a built by hand environment can be very diffiult to replace. Especially if they are the original architect of the systems. I think this is why you get problems like the UK banks recent hickup server side (no one was left who had an adequate view of the complete systems).

I like to have a mix of old and new hands as well. The new learn from the old and can get a decent apprenticeship and the new can think differently and a lack of knowledge may mean they try things others would think impossible or just think completely differently. Plus if an oldie leaves an apprentice is in a good position to take over.