To be an "heir" is not a caste-rank. As subtle as it might be, there is individual choice in this society: the head of household's choice, in this case, of whom to appoint as heir. Sometimes, there are mistakes. Being poorly suited to run a household does not mean an heir cannot be a suitable noble in other respects. The heir is already a noble, just perhaps not a noble suited to running a household. But Ai-Naidar in the noble caste also serve supporting functions to Heads of Household.