If you remain perfectly still, you’ll stop sinking at your waist

I’m not sure you understand buoyancy. Floating objects float, they don’t sink no matter how much thrashing about. And while quicksand is denser than water, it’s still not dense enough to float a human at their waist, it’s actually around their chest.

Nobody dies from drowning in quicksand. Ever. It’s always dehydration/fatigue.