The most important issue.. is the need to prevent bacteria or for that matter any bacteria or food partickles to reach the bottom of the tooth, meaning the end of the root. Particles can enter through a fissure in the gum - do called pocket- or by way of toothdecay. Preventing such is needed to avoid a root infection. There are near no options - pulling a tooth or fill it. the filler used over 80 years is guttapercha. and a crown will need to portect the tooth from crushing from biting on it, most often it will be gold and plastic over it. -- Once a tooth is pulled the space needs filling. Putting a screw in the gum is one way, wearing a bredge is another. In all cases plastics and gold and titanium will be used. Filling just a superficial cavity today most often uses moldable plasic made to harden with a small blue light. Until they find new materials.
Taking the mercury fillings out can damage the lungs more from the dust inhaled.. as long as they are ok, I would leave them. When they need taking out is time enough. I am not so all certain that plastic fillings are healthier, but they look better. In old age we will have white teeth again. and have metal joints.. haha.