You are comparing an almost 40 year-old technology that has seen billions of investments in research and implementation with a technology that in real terms is barely 10 years old.
Also, there are several systems from Avaya, Cisco, Nortel, Siemens etc that can do most of the stuff you describe, and many others that circuit-swithed systems can't. And that is the ONLY REAL difference between VOIP and the rest:
VOIP switches packets.
Traditional telephony switches circuits.
Other than that they are both "digital", microprocessor-based systems. Also, it is not true that all traditional pbx/ksu systems are client-server. KSU-less systems are peer-to-peer. Some of the older pbxs were following the mainframe-dumb terminal model. Btw UNIX or Windows has nothing to do with client-server, it is just a model they may or may not use. They both use peer-to-peer, client-server, or mainframe-dumb terminal when it suits them.
Other than that, you make very valid points. The thing is that most companies already have infrastructure in lan cabling, lan switching etc. So often a new IP extension can be as simple as plugging in a phone to a lan jack.
