Okay, this will be difficult to parse with [REDACTED] everywhere, so there will be MAJOR SPOILERS FOR THE SERIES 8 FINALE IN THE REST OF THIS POST.
I MEAN IT. MAJOR SPOILERS.
YOU SURE YOU WANT TO KEEP READING?
REALLY?
DON'T COME WHINING TO ME IF YOU KEEP READING AND GET SPOILED.
LAST CHANCE TO BACK OUT!
...
Okay, here goes:
I personally never felt like the John Simm Master was a real threat, even when he had ostensibly "beaten" the Doctor. In fact, the last Master I felt was a true threat (outside of Big Finish)? Roger Delgado. And even his level of threatening...ness... diminished over time.
Now, Michelle Gomez? What she did to Osgood immediately made me take the Master seriously again in a way I never did in Series 3 or The End of Time. Sure, I was very sad to see Osgood go, especially so matter-of-factly, but I really do think it helped solidify "Missy" as a real, honest-to-god danger to the Doctor and, well, the whole world.
I completely understand being upset at the character's demise, and I'm not trying to say that you shouldn't be. I'm just saying I don't think it was wholly unnecessary - in fact, Death in Heaven woudln't have been nearly as effective for me without that scene.