Listen up, jackass. I was a voice major in college and have been a professional singer for 40 years. I know more about singing than you know about anything.
Whitney yells from her throat and her chest. Her switch to her head voice, when she uses it, is the most obvious in the history of the pop charts. She yells so forcefully that her pitch suffers. It's all over the place.
She has been singing incorrectly for so long that she can no longer sing. Great singers can cut it into their sixties and seventies. Whitney hasn't even turned 50!
Her ruined instrument isn't the function of substance abuse. Pavarotti and Sinatra smoked...the worst thing you can do for your voice outside of gargling with acid...and held their own for decades past Miss Houston.
If you think this woman is a great singer, you're stone-cold deaf...and an idiot.
You're out of your mind. I don't care what you've been taught in school about singing. They simply don't get much better than Whitney Houston. As for your alleged 40+ years as a professional singer, there are loads of people who can't sing worth a shit who've doing it just as long. Bob Dylan is one obvious example.