You said "most" of your friends, and not "all" of your friends. I take this to mean that some of your friends are not perfectly happy and healthy.
But, you still haven't answered the question, as to which criteria you use to judge whether these people are perfectly happy and healthy. I personally doubt that this is knowable, since people can appear to be perfectly happy and healthy, when in reality they are neither.
The point is not debatable, that mercury is a neuro-toxic poison. Those who believe that sucking on neurotoxic poison embedded in their teeth 24/7 for decades is harmless activity are truly amazing people.