Being a holding company which rightfully acts on advice of the principal shareholder Tata Trusts, they need to have Noel Tata to preserve the identity and heritage of the Group. Anyways the group companies are managed by professional Managers. Let the Tata Sons Board act as th custodian of group ethos and shareholder interest only. And none more suited for that than a Tata himself.
normally when a leader is changed the new leader should be superior or at least at par . If it is inferior( mostly parachuted) , then existing leader has to have time, skill and patience to bring up the new leader at par .. If this fails then successor has to go . this is what has happened.