What happens if a tooth decays inside the artificial crown with time?

Maybe tooth loss. When a crown falls off, if it is decayed under it, the decay must be removed. The crown can sometimes be put back in, other times a new one must be made. Sometimes the tooth must be removed. Replacement with an implant supported crown is recommended. This happens less often with new all ceramic crowns, since xrays pass through newer crowns. You cannot see decay under crowns with metal.
Bad things. Is a given in dental surgery that all decay in a tooth be removed prior to placing a crown. From that point on, the only decay that can occur is from the joint between the crown and the tooth. Bacteria. If allowed to concentrate in dental plaque will break that joint through creation of acid and demineralization of the tooth. Floss and brush daily. Also chronic acid saliva can break the joint.