In the pre-VoIP days, PABX manufacturers made their money from the line cards on their systems. (2K upwards for a card which was just a 32-channel 8KHz ADC ? WTF !) And if you wanted a phone which had a basic menu display (so you didn't have to remember cryptic codes to activate features) the digital line card cost even more. Then there was the price of the digital handsets themselves.
Now, in the era of SIP VoIP, manufacturers realise that this revenue stream is under threat. You don't need specialist hardware to run a phone system, except maybe to interface to traditional telephoney (ISDN, POTS) All they're selling you is software (Especially the management side of it). So they need to lock you in to their ecossystem. "Of course you can use a third party SIP phone: But the license to configure it onto our system will cost twice the license for one of our phones (Which cost twice the price of your cheap SIP phone). Oh, you want us to talk real SIP, not our special version that only our products support...?"
Telephoney manufacturers are in real danger of shooting themselves in the foot. A Windows/Android/iPhone smartphone (OK, not the most expensive ones) can be had for less then the higher end corporate VoIP handsets, does a darn sight more and gives the user the street cred of having a fancy smart phone rather than a bland corporate phone. Mobile companies are also offering quite interesting tariff deals now (free line rental and free calls between mobiles on your corporate contract) OK, so coverage can be a problem, I agree. But with a mobile, *you* have no infrastructure to support. (Unless you buy some pico/femto cells to boast coverage - and they just plug into your IP LAN, so just another dumb endpoint)
But the bigger problem for PABX manufacturers is utilisation. People just don't seem to be using phones any more. Email, IM, Facebook/web & Skype are what a lot of people are using to communicate nowadays. If you're the Finance Director, and you walk around your building and see phones just gathering dust and not being used, you have to ask the tough question: Do we *need* an internal phone system ? It'll be a brave Finance Director who says "Let's not have a phone system" but I can see it happening in the future.
I've worked with phone systems for 20+ years, on systems in size from 20 to 20K end points. I can see the writting on the wall for my profession.