Darran, please read my post on ratings comparison. Doctor Who is still one of the top rated dramas on television full stop. Television ratings in general are down across the board, mainly because streaming services have gobbled up viewers. However, Power of the Doctor was number 5 in the TV chart the week it was broadcast, which goes to show that Doctor Who is still popular as ever with viewers.
I've been looking at the viewing figure data for Doctor Who to make some comparison with other big TV dramas. The final series of Peaky Blinders saw an average 5.42 million viewers over its six week run and that's pretty close to Doctor Who: Flux's average rating of 5.43 million viewers. And yet Peaky Blinders is heralded as a success, whilst Doctor Who as a failure by some because of the viewing figures. The reality is, Doctor Who is holding it's own against the big dramas.