It's strange. Physically, Sharapova is nearing the end. Her body is breaking down...but that desire and that competitive fire is still there. If she can trust her shoulder, I think it will be easier to hold her serves. She can beat top players on a good day and still seems motivated to win a grand slam.
Vika and Caroline just lack that spark that Sharapova still has. They may be younger, but less likely to ever do anything big again.
Sharapova is a champion and will probably fight on and keep believing she can win...like, truly believing, not fake posturing like Vika seems to do for 1 set. Caroline's game is not a champion game.
Maria has the biggest fire and desire to win and play tennis and is still hungry for success
Yes, the problem is her body. The many injuries. Not her strong mind.
Maria loves the sport tennis, the competition.
Azarenka still has fire. But her son is priority #1 now.
And as we have seen vs Halep she breaks down mentally. Choker, nervous, mentally weak.
Her many easy UEs from 3-1 were shocking. No consistency anymore.
Caroline's fire/motivation for wins is not that big anymore as it used to be.
She has won her Grand Slam, has a bad illness and found and married the love of her life.
All these things stopped the big motivation/fire/hunger for wins she had.