capaldi felt like the absolute end of where you could take the character as a male. a brilliant actor, probably the best man the show will get in terms of the respect he garners, raw talent and experience
not to mention to swell of cultural opinion that was demanding the change. she's a fabulous actor with so much more to give. this is the thing, so many minorities and women NEVER get the chance to have a meaty role and show their range. whittaker will blow people away.
also the show has always had problems with consistency. people remember tennant fondly but he had just as many if not more wonky episodes. but around 2010-2013, standards in TV changed drastically and its flaws became more exposed. and of course there was a change in how moffat handled arcs and accessibility.
on the other hand, find me another show that's more eclectic in tone and genre. it's the nature of it.