Also (about the spin off) They barely manage to come together to film the actually series, having a spinoff would do nothing but be a waste of time and resources that could go into making the actual show.

Well, I guess the idea of doing a spin-off involving supporting characters would be that hopefully these actors are less busy than those playing the lead roles and would therefore have time to fill in the gap? But Moffat & Gatiss don’t, with Dr Who and all!