DW has been sort of up and down since 2005. Definitely suffered under both Moffat (who I think was more suited to single episodes like Blink - arguably one of the best episodes since 2005 - rather than ongoing story arcs) and Capaldi.
I really hope Whittaker will be what the series needs to revive it. I think it will be really interesting to see a female doctor in action, and Whittaker is a very good actress. I remember reading in the magazine a few years ago that it's totally within the realms of possibility, especially considering that it's literally about a time travelling alien who has had like 13 (believe Whittaker is #14?) bodies in space. I'm really looking forward to it.
However, it is a little suspicious that for the first time in the like 50-something years since the show first aired that it's been moved to a Sunday. Might be nothing but does kind of feel like if the ratings go down because it's on a Sunday night when people have stuff to do then it might be something they cite as a reason they should switch back to a male doctor.