I think that any node on hive blockchain should have the potential to work as a master node. No specifically the witnesses, whale, or whoever.
But for work as master node should have a portion of HIVE stacked. So it opens an away for those that just want to support the blockchain with this function. It's open a way for those that want to invest in the HIVE blockchain specifically for that function. So If someone that wants to have HIVE blockchain master nodes could have it. And I guess that it makes more decentralized.