It's like changing the decor and function of a "locals" pub. (Marco will know what I mean).
"Regulars" will be paying customers come rain or shine. You won't be making a great deal of money, but you know roughly what they'll bring (low viewing figures in this instance). IF you plan to change the entire function of the pub (into a wine bar say), you run the risk of losing your "Regulars", in exchange for attracting a newer, younger, cash heavy demographic. That's fine if your plan is fool proof, but if your "Regulars" feel threatened and stop coming and then you can't delivery this great wine bar, you'll lose the potential new customers interest as well the bloodline of your old establishment.
It's always a risk and if your biggest fans (Dr Who) walk away, it's set for failure.
Personally, I think it's a move that was necessary, rather than the notion of the series bowing to the PC brigade. The series as a whole was getting bland and uninteresting, if this is the kick up the arse it needed, then so be it. Personally, I think it should have happened in Capaldi's place...the older Doctor thing for me felt like an embarrassing Uncle. Problem now for this Doctor, that this role is potentially "cursed", thanks to the "13th" label.
I hope it does well. I'm NOT a watcher as per se (Only seen about 10 full episodes altogether, from Christopher Eccleston's Doctor reincarnation, including a handful of Tennants and absolutely none of Matt Smiths'). So as a neutral observer, I think this move was required.