Re David / Pip: because we don't, in RL, have to keep reminding listeners of our familial relationships and grandparents' names, I'm not sure a direct comparison is helpful. (The dynamic that people are objecting to is complicated / compromised by radio production necessities - but I don't think that is the cause.)
But just consider other families with grown up children in the village. (Not sure if examples need to be parent and child of the same sex, but since we're talking about David and daughter ...)
Neil and Emma. Lovely close, affectionate, teasing, humour filled relationship. They never sound as if they're having an affair.
Tony and Helen. Poor man is exhausted with her alarms and excursions. She's rude and patronising. They rub along.
Clarrie and Will. Close, mutual support. Constantly striving to please each other no matter what. Creepy? Never.
Brian and golden stepdaughter Debbie. Close, brisk, been through the mill, equal measures of trust and distrust. But we've never once speculated that he was sneaking off to Hungary because he preferred her to Jenny.
The difference is, in all those instances, you can hear the years and years of worried parenting in the voices of the parent, and years and years of rebellion, dependency and growing gratitude in the voices of the grown up children. Each parent (perhaps I'm extrapolating too far) sounds as if their pride in the "child" is bound up with their love for the other parent, who has helped to build that child.
For some reason this is all lacking in the David / Pip thing. I don't know if it's because, with a new Pip, they just haven't had time for all the parent /child stuff to grow into their voices. David used to sound constantly exasperated with old Pip. Now he always sounds as if he goes to Pip for support and reassurance and that they are the primary "team" facing up to the vicissitudes of life with Ruth. Wringer than a wrong thing.