I'm not sure what proof there is than phablets are irrational as you've stated.
I got a Note 1 phablet after my iPhone 4 as an experiment. It was an audacious device where Apple had gotten stagnant. While the galaxy note 1 was a premature priduct, the phablet form factor was a constant constant piece, and it nearly ended the use of my iPad. My android phablet remains in my life/because Apple missed the boat on screen sizes and is playing catchup.
The phablet phenomenon is real and a calculated tradeoff.
Phablets have cut into other markets and for most people globally if a movie is the Mir primary computing device, the phablet is more appealing. In the case where summertime doesn't use a phone much, they often have a tablet, laptop, etc.
Not having a headphone jack remains a pain, Bluetooth ear buds need to be at 20 hrs of battery life to get wider adoption.