More extensively damaged teeth from decay or failed fillings can be restored with a crown which covers what is left of the tooth. If done well with careful preparation and using a high-quality laboratory the life and function of the tooth can be prolonged many years.
Acrylic or plastic dentures are supported on the gums only, though not ideal, can maintain good function for many years. Cobalt chrome or metal dentures are supported on some of the remaining teeth, allowing them to be thinner and smaller.
If left untreated decay can cause the nerve/pulp to die and lead to pain and infection spreading out of the tooth. Treatment is to clean, disinfect and seal the roots which can prolong the life of the tooth for many years.