I think this debate needs widening out, maybe the question should be, "When is a phone a phone, or is it a PC, and who cares, if it works use it".
I am so looking forward to the day I can have calls come on my PC, Surface (Gave up on laptops) and mini tablet (Phone) and answer them irrespective of device, in effect I can do this with Skype and I believe this is where Windows 10 is going phone wise, the PC doesn't need a phone chip in it, It can just piggy back call using the mobile like I just replied to a text using Cortana on my PC, the whole concept of telephony is changing, it's all going VOIP, add in the cloud and the device we call a mobile changes.
The HP device is in many ways a game changer but in truth it's still a phone with big capabilities but as the OP said, this is just the beginning (Well to be more accurate Windows 10 is the beginning).
Although that isn't really a true statement as all of it is just a continued development of previous technology, but you get the point.
I still have a desktop, I have two tablets (Probably give my HP Stream 7 to my boy this weekend so down to just the SP3) and my 1520, this will be replaced by whatever I consider to be the best phone running W10 later on this year (Could be the HP, Surface phone or whatever other nice stuff turns up). By then I hope the new Skype apps have a lot more functionality, so I can call from a PC, or do PC stuff from a phone and switch screen size with ease without having to carry them. So with VOIP/Skype, Universal apps and the cloud, I can turn my lights off and on from all devices, check my heartrate, turn the volume up on my TV, but if someone calls me I am stuck to just one device, makes no sense to me.
As they say in the ads, "the future is Orange, probably with a lot of Gin in it, brighter than you think and MS and friends are on the right side of the ball park".