I work with phones for a living at the largest private employer in Philadelphia.
While office phones are clearly on the decline, they ain't dead yet. We have approximately 20k phones, half of which are VoIP and half of which are either POTS or a digital offering from the local carrier. All of them are converting to VoIP, slowly, and in the process I'm watching the attrition that the OP probably expects. It makes sense to get rid of single lines where they're unused and unnecessary.
However, there remains the complex office setup where you have administrative assistants, or a suite front desk, and shared line appearances. Once someone wants to be able to put a call on hold on one phone and pick it up on a different physical phone, they want it to work like the same technology did in the 80s.
Of course it was easier in the 80s, when those phones shared a dedicated physical copper pair that carried nothing but the voice. With digital signaling it's significantly trickier; Broadsoft has a proprietary protocol to handle this, and the IETF specification (http://tools.ietf.org/html/draft-anil-sipping-bla-04) never left Internet Draft status (which, frankly, is a good thing as it's a very poor protocol).
I don't see that complex setup going away any time soon, as it's a common VIP pattern.