A pixel is the smallest controllable dot on a screen. The spec shouldn't have changed the definition of this, but introduced a new unit instead. Personally, I'd have both!
Everyone was already using the existing "px" unit because UAs had shipped it. Would you have preferred that every single web page broke on high-res devices (so that no manufacturer would ever introduce any high-res devices to the market) to the status quo?
What would you have used the "device pixel" unit for, exactly?
What would you have used the "device pixel" unit for, exactly?