My understanding, whiich I think came from articles in Doctor Who Magazine, is that the ratings took a nosedive during the Capaldi era, though a lot of people like to blame the Whittaker/Chibnall era. For viewers who started with Tennant or Smith, a significantly older and harsher Doctor was not what they expected. Meanwhile, the age of BBC iPlayer in the UK and Netflix had begun, which started to affect the ratings of a lot of broadcast shows.
I used to do the monthly sales charts for Diamond Comic Distributors, and I have an anecdote about the Capaldi Collapse.
Titan Comics, after they picked up the
Doctor Who comics license from IDW, was initially quite successful with
Doctor Who comics. They had an aggressive plan -- ongoing monthlies for Tennant, Smith, and Capaldi -- and each of these titles routinely sold in the 10k range each month. For a licensed comic, that's a good number!
After a year, they did a multi-Doctor crossover, Paul Cornell's
Four Doctors. (There are actually five, arguably six Doctors, if one counts the alt future twelfth Doctor as a different Doctor.) This sold very well, about 15k an issue.
Comics in the direct market are sold on a non-returnable basis, so stores aren't generally going to order more of a title than they know they can sell. Based on the numbers over Titan's first sixteen or so months publishing
Doctor Who comics, there was clearly a market.
The market evaporated after
Four Doctors in the autumn of 2015. Titan's titles were selling at much lower quantities than they had been before the crossover. Partly, that was Titan's own fault; they added an Eccleston series and various mini-series, which would bite into consumer dollars, but it seemed pretty clear to me, just from the sales figures, that the Capaldi era was turning people off to
Doctor Who.