the implication that he is never going to come back and that he’s been replaced by a human dad

I took it more as him knowing he was going to stay away during Poppy’s childhood for safety’s sake, and that he would have a different face when he returned.

I wanted to add that Thirteen’s appearance has really grown on me - I think she’s the one incarnation of the Doctor who would be able to drop the sanctimonious “I have to stop you” bullshit so quickly, and help Fifteen instead. The others would have gotten there eventually, but she’s the version most equipped to get there so quickly.