kag - who is to pay if they get sick, if anyone gets sick with a life threatening illness?  the point of insurance is the spread the risk.  If we spread the risk among all people, then the average any one pays is no more than the fair share of all the expenses.  In other words, rather than one person being financially ruined by ill health - the point of insurance - the cost is spread among everyone.  Break this down into smaller groups and the risk of catastrophic costs hitting someone in that group rises; when that happens that groups premiums skyrocket.  More, put "healthy" people in one group and "unhealthy" people in another group, and it is a foregone conclusion which group will have higher expenses.  

By the way, I am also on Medicare and find it good.  I do have a Medicare Supplement plan; I have to hope my finances hold up so I can keep it.  If I can't it would not take long for a serious illness to deplete the rest of my assets.