Letting Doctor Who die a natural death at the end of the 13'th incarnation may be a simple matter of practicality for the BBC. Just like in 1989, the BBC (remember, owned by the British government and run like our country's PBS) simply can't afford to produce it much longer (hence this last split-season). At least not to current US sci-fi TV standards, and certainly not to longer than 13 episodes a season. The country's chronic economic woes and government budget crises won't allow it. Finding a co-producer isn't an option either; trying to find one was a large part of the reason Doctor Who stayed off the air for 16 years (minus that one-off TV movie which Universal chipped in for).
If I was a BBC exec, I'd be thinking right now "The sooner this thing ends, the sooner we're freed up money-wise to produce other shows and the sooner we can farm it out to Hollywood for the JJ Abrams treatment and rake in the profits from the licensing fee." The 13 lives limit, and inserting John Hurt as this lost Doctor, moves the saga very close to Trenzalore - to the point where the Beeb can end it without facing a backlash from the fans (as they did in '84). Yeah, they'll still grumble, but the novels, comics and radio dramas continue to be churned out on autopilot for next to nothing and will continue to do so for years to come.
First and foremost, the money invested in making Doctor Who is made back with extra from selling the Series (Unless things have changed this half Series) and although the ratings have gone down, everyone's ratings have gone down, hence why the share hasn't changed.
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?