I'd agree the writing was much poorer, but I do think there is an element of there is a requirement on the actor being able to bring it to life and make it watchable.
We were addicted before, but watched two or three episodes with capaldi and gave up, sad to say, so I imagine ratings are way down, which is a mix of the wrong lead and poor writing. It's a shame as he is a brilliant actor.
I'd agree with the poster who said they changed this to appeal to a certain demographic and when they put an older doctor in it messed with that.
In addition, I've got to be honest, as there was always some unspoken romance between the doctor and his assistant in the new doc who, I found lthe whole older man younger woman a bit creepy. Although they made sure to not make it romantic, it just looked wrong in my view as a pairing. The visible age difference was simply too much.
All in I don't think capaldi was to blame, I think they just got a number of elements wrong and it was pretty much always going to alienate their core demographic.