Honestly, Matt Smith was made to be the last regeneration because Moffat wanted to solve the problem himself. He’s introduced so many characters and ideas that are intended to be the most important things in the Doctor’s being (River Song, Clara, “that sound is the parking brake”, regenerating into different races/genders, the 50th, etc. etc.) that it’s only natural he would want to tackle that problem too.
I’m only speculating, though.