Good point. I guess as well that BBC series length has always varied somewhat, in fact aside from Who Torchwood and Merlin (and I guess Robin Hood may have been the same but I never watched it) it’s hard to think of another show I remember having a 13 episode stint since Blakes 7 (though I’m sure there must have been plenty) and it may just be that we need to get used to a truncated series length, I mean there’s nothing that even says a BBC drama always needs to have a set number of episodes per season, Spooks, as I recall, varied greatly from as low as 6 to a high of about ten episodes, and even though the examples you’ve given all have 8 ep runs, Silent Witness still gets 10. Plus of course Torchwood varied greatly too (13,13,5,10)
I suppose the only thing to point out is that Call the Midwife et al tend to be 60 rather than 45 minute shows, so 8 episodes of Who means less screen time than 8 episodes of Call the Midwife say.
I could live with 8 episodes a year plus the Christmas special, you’d have to hope that might filter out the crappier episodes but it might sound the final death knell for 2 parters (we might still get one). Part of me thinks the BBC should at least come clean about this, but I’m not sure they do likewise with other shows so it might not even occur to the higer ups that this is any kind of issue?