Re: Osgood and Missy are not entirely "dead"
"Or you know, they could use something totally off-the-wall for Dr. Who, like, I dunno, perhaps time-travel?"
Thus making any emotional investment pointless and tugging the audience around pointlessly, rather spoiling immersion.
Ultimately, any writer can save any character with a little adroitness and cunning. However, Moffat lacks that delicate touch and his 'twists' are pulled out of nowhere. That's not clever writing: It's like running a shell game without showing people which shell the pea is placed under in the first place or shuffling about. He lifts a random cup, goes 'ta-da' and expects us to be impressed.
Chekov's gun is not clever if it is hidden in a desk draw where nobody could see it.