All I'd say is that whilst it was a shame that IMO a strong series saw the ratings drop, I personally don't buy the idea that Who has suddenly ceased to be 'mainstream'. An average of 6.7m (Just short of 8m for Xmas) is still very decent for a Drama series in 2016. Who doesn't exist in a bubble (Universe) and considering the length of time it's been on, I really cannot see the BBC worrying too much at present. (Now if there's *another* drop in 2017....maybe. Ah, a trend. We'll see.)
I mean, the Soaps regularly get figures ranging from 5-7m these days compared to, say, 5 years ago, and I assume they're still considered mainstream!
I know it's a different set of circs and long long time ago (So long ago I wasn't alive/very young!) , but if you look at the first 10 years of C20 Who, the fluctuation in viewers is significant. Steady start, then a real spike during Dalek-mania, then a steady dip during the later Hartnells, then a spike with Patrick, then a dip, then a spike (from Pertwee's Second series on, really. Inferno dropped to about 5m...yeah, that Classic!) then.....etc. These things happen with most TV Shows over time...why should C21 Who be any different?
Having said all that, I stated months ago on another thread that I think we'll see a regeneration at the end of 2017. I'll be happy if I'm wrong, but if I'm right....well, we'll see what the next Doctor's like and how he (?

) goes under Mr Chibbers. Although the assumption that a new Doctor/Showrunner automatically making the show buck the trend of TV ratings in general/be immediately popular and keep the initial curious casual viewers etc is just that, really. An assumption. Or hope!
A hope I share, mind. Quality is the most important thing, though, and obviously that's a purely subjective thing. But any Drama that averages over 6m these days (never mind taking into account viewers/sales worldwide in Who's case) ain't going anywhere any time soon.