Official comment
They're two completely different products. Yes, they may share some features (they both have 4 channels of mixing, on-board effects), but as you note yourself - there is also a difference in the audio quality; the DJM has better components. The DDJ offers an all-in-one solution for control and mixing, whereas the mixer would require external devices such as turntables or CDJs for input, whether it's audio or control (to manipulate software).