You assume that balrogs can only be stopped by unmined bedrock. Since the chance of a given balrog being stopped by bedrock but not by the combined efforts of the dwarves is muniscule compared to the chance of a weak one that can be stopped by mithril-clad soldiers or a strong one that can dig through mere stone, the best defense against balrogs is to mine and guard the mines well.
You assume that balrogs can only be stopped by unmined bedrock. Since the chance of a given balrog being stopped by bedrock but not by the combined efforts of the dwarves is muniscule compared to the chance of a weak one that can be stopped by mithril-clad soldiers or a strong one that can dig through mere stone, the best defense against balrogs is to mine and guard the mines well.