If I had consistently been scoring 87% in my end of year exams and suddenly dipped to 83% one year, I wouldn't be too worried.
I doubt Moffat will be losing any sleep over this, a 3-4% drop is miniscule.
The AI's are tricky things. As has already been said, End Of Time was very highly appreciated by the general audience, but objectively, it was a bit of a mess and got mixed reviews both from the critics and within DW fandom.
Moffat has to tread a fine line between writing things that connect with the general audience while keeping his artistic integrity and writing things that he thinks are clever, involving and challenging.
If the AI had dropped some way below 70, I'm sure Moffat would ask himself some searching questions and perhaps change direction slightly in order to win back the casual viewer. But a drop of 3-4% really makes no odds, and Moffat will feel no pressure to change direction. Series 8 is clearly still connecting with a lot of people and it's also been very highly regarded by both critics and large sections of DW fandom.
If it ain't broke.....