I saw a bit of the Porridge episode. I thought it was rubbish without the great Ronnie Barker. I don't know why they're bothering. Write some new things.
I think Goodnight, Sweetheart had run its course. What more is there to do in that?
Ultimately it's the writers you need to convince. Perhaps they feel they have no creativity for the series any more and don't wish to create a sub-standard series to spoil the full set.