Its peculiar as the series is obviously a hit and the BBC seems to position it as a flagship but commission very small amounts of it. I don’t know if the limitation is the directions the writers are able to write in, without it going outside of story and characters, or Elba’s availability.
Your classic American long series has the obvious filler episodes, you write the story arc for the series and work out how to get from A to B to Gets the Girl, if it goes too long you get A to B C,D,E,F Who was that why are they there etc. before getting bored and not caring about the ending.
The best things leave you wanting more, the rest make you think they put one too many episodes in.