In a nutshell, my Views are that Tiffany was introduced to replace Granny, who Pterry is going to kill off, and that Sam Vimes cannot be sustained as the literary device he currently is and will therefore have to be killed off, which is why Pterry introduced Sam Vimes Jr - because if you start with a character from when he was a baby, that gives you a lot more time to write about him than if you start from when he's in his late forties and suffering heart trouble.