Most of them have been replaced. The ones I have left are probably more than 30 years old, and when they start showing clear signs of deteriorating, out they go -- but not so much because of the mercury aspect, but because I don't want bacteria getting down through the cracks in the filling and causing problems which could eventually result in a root canal and a crown. That it gets rid of mercury in my mouth is a bonus. A new filling is a LOT cheaper and a lot less painful than all that.
Did you know that when a composite material "cures" i.e. transforms form liquid or doughy, to hard, it shrinks. Yes, it shrinks.
So, you fill a hole with composite, shine the magic light on it, and the material shrinks, do you think this might cause some problems?
The problems this shrinking can cause are:separation of the restoration from the tooth,
separation, or strain between the bottom of the restoration and the tooth which can result in a lot of sensitivity,
Now, these problem can be overcome, but over the many many years dentists have been trying to make this material work as predictably as "good old amalgam", these issues have continued to plague those who use them. I can absolutely promise you that EVERY manufacturer of composite systems, and the required adhesive systems that are used with composites, are marketing new "generations" of products, over and over again, and have been for decades, all promising to alleviate the basic issue of polymerization shrinkage. I could not count all of the lectures I've attended where this problem was proclaimed to have been "solved", only to learn at a subsequent lecture that once again, "it has been solved."
This does not mean that your dentist can't create a great, well serving composite restoration. But it does mean that the use of composites, especially on back teeth where you are going to exert large crushing forces, is extremely "technique sensitive", which is dental speak for "very easy to screw up".
The typical life expectancy of composites in back teeth in functional positions is way less than amalgam.