It is not because it has the fastest processor. It does not.
It is not because it is the smallest. It is not.
It is not because of the design (Dare I say this?). Everyone can copy it.
I think that the secret sauce is that IPad is created at the right juncture between unrealized consumer needs and technological advances. What is the nexus with Pentax?
Bottomline is that Q's small sensor may not longer be a major shortcoming given the technological improvement. Counter-intuitively, Q has the potential to be a success story. FF's advantage is increasingly diminished and will become just a status symbol in a foreseeable future.
With Q and 645D, my guess is that Pentax will not venture out toward FF, being mirrorless or not. My recommendation for Pentax: stay put and make existing products better.