Putting phase detect on the image sensor is on the way to solving the AF problem for mirrorless, and Canon's DPAF is another big step in that direction. Contrast detect is slower because it's more iterative, phase detect determines direction and magnitude at the outset.
You're correct about the Leica rangefinders. No one complained about the film versions, because film is not affected by the incident angle of incoming light. No one complains about the digital versions, because Leica took sensor and lens design steps to compensate for the problem caused by high incident light angles with a short flange distance.
Speaking of rangefinders isn't a big issue with on sensor PDAF going to be that the light is spilt over a very short distance compared to a DSLR? rather like the difference between a rangefinder with a short and a long baselenght.