Stevie's Voice Decline in the 80's

[The Juggler]

I hope people don't take this as "Stevie bashing" because I love the woman and her music, but this topic is something that always interested me.

I love this demo of Secret Love from 1980, her voice is so pure and clear in it. It's beautiful and the effects of coke haven't been seen yet.
https://www.youtube.com/watch?v=OltUuI0kv38

Skip forward a few years to the Mirage tour in 1982, Stevie's voice is still there but it's rough around the edges. But it's still her voice.

https://www.youtube.com/watch?v=eBS-Z_CPuMo

Even on the Wild Heart tour, her voice is very rough but it's still completley recognizable. So what happened between 1983 and 1985?

Compare the best few notes of Beauty And The Beast to the opening shriek of I Can't Wait...it's a completely different voice.

So what caused this complete change? Was it the coke, the hole in her nose, the ciggarettes, what? And how come Christine or Lindsey's voice never changed, even though they both smoked and did coke also.

By 1987, her voice was completely unrecognizable to the pure, sweet vocal on Secret Love from just 7 years. What happened? Was it a combination of factors like drugs, and perhaps wearing out of the voice?

I know as people age, people's voice's go down in register, but Stevie's has changed completley.

Again, please don't take this as Stevie bashing. I love all of Stevie's solo work, and I love her voice but it's nothing compared to her beautiful voice circa 1977-1981. Why do people think her voice changed so much?