really don't understand your comments about not having enough game. To say you are amazed she has gotten this far is actually amazing to me.
When Key's game is clicking, she can beat anyone. While not the most agile, she has plenty of game elsewhere to win a major. Her major issues are lack of strategy on court much of the time, and failure to string together long stretches of consistent play. She tends to go through spells where she hits way too many errors and cannot put together a consistent strategy on the court.
Since she has shown, just recently, she can beat the very best (just beat Muguruza this summer and should have beaten her a second time), it's all about stringing together successive matches where she is playing her best.
Now I'm not saying she doesn't have major issues to work on to become a consistent top 10 player, she does. But she has way more than enough game to win a major, and has clearly shown she can beat the best in the world on any given day. So not sure why you seem so befuddled by her reaching the QF here. She is only 22, and still has plenty of time to continue to improve. And we've seen plenty of times where players less talented than her have gotten hot at a major to win it.
I think many of your points are valid in terms of what has held her back from reaching, what many thought, was her potential a few years ago. I think many thought she would be a top 5/10 player by now, and have threatened to win a major. Her less than great movement, her lack of strategy and her consistency issues are exactly what have held her back from reaching the peak of women's tennis. But she has plenty of game to win a major, no question at all.