Digital Spy has a great article debunking some of the 'beginning of the end' ratings talk - what they ultimately say is that ratings have definitely and definitively fallen in the UK by around a million, but it isn't as bad as the overnights would have you believe (as much as 4 million) thanks to time-shifts. So it's down, but not as down as the doom cries would have you believe. The worse thing, they argue, is that the show is now losing steam in terms of its perception from the media and from the general public, and thus ends by saying possibly the best thing for the show now would be a rest that perhaps the time is coming for 2-3 years off and a new outlook/showrunner when it returns.
Incidentally, this is precisely what the BBC wanted to do when RTD told them he was planning to leave, but RTD fought back and essentially forced the issue and said "you have your successor" pointing to Moffat and even doing an extra year over what he/Tennant/Gardner initially planned (the specials) to give them breathing room, so I could absolutely see it happening in 2017.
I really think Capaldi will stick to Troughton's infamous rule - advice given to Peter Davison then passed on from him down the line - do three series', then go. Tennant followed it, Smith did - both of them cited the Troughton Rule in interviews. Can't see Capaldi doing differently. If we get 4 specials (plus Christmas) in 2016 as the rumours indicate, then one more series with Capaldi/Moffat in 2017, there's a big question mark of what happens beyond that. We know 10 is the last Series Moffat is currently contracted to do as well. It might be time for a short rest.