I worked for an ISP for a few years programming the phone system (using Nortel phone equipment) for the call center and I agree with your many points. I self-taught myself how to program the PBX, CallPilot and Third Party software to connect the Nortel equipment to Dialogic cards. The company recently set up another call center out of town with VOIP phones connected to the PBX and the calls get constantly disconnected because of QOS. The company did not want to spend extra money on a dedicated internet connection between the two offices.
Will Asterisk overtake the traditional PBX? Not until the QOS issue has been resolved. But then all major advances in technology always had a steep curve at first anyhow.
And just to let you know, being an IT guy, I've taken down the PBX several times a week some times :) I probably should have gotten fired about 20 times!
I'll feel badly for those who invested quite a bit of time and money getting the proper certification in traditional PBX training only to find that the open source community has robbed them of their livelihood (when the traditional PBX eventually loses it's market share). On the other hand, when life hands you a lemon--make lemonade. A PBX technician could use the opportunity to learn the new technology as well.