4K at 15fps isn't as great as it seems. Secondly, Canon DSLR's are not video cameras, they are still cameras with a video feature, there has to be some balance between the stills/video. And yes the new GoPro looks great and I'll probably get one, but you can't expect the same IQ out of it as a DSLR. But the GoPro has a tiny sensor, it's much harder to get high frame rates out of a full frame sensor. You want high FPS with a larger sensor, the Sony FS700 is about $8k and does 120fps/240fps@1080p and 960fps at low res.
Specs and numbers aren't everything. It's like saying "oh well this T4i is 18MP and the 1D Mark IV is only 16, why is the 1D such an overpriced piece of crap?" When we all know that the 1DiV is better than the T4i in every way. And while it's nice to have high frame rates available, how often will you actually use them for a project (not just messing around)?