I think BNB can't replace the Bitcoin for being the King of all coins and even ETH. No doubt that the BNB's potential is there and for being almost on top in the market and offers small gas fee compaired to Ethereum. Many people still choose bitcoin because it has been known for a long time. Companies, stablishment and others already use bitcoin or accepting BTC for their transactions.
If we talk about replacing bitcoins I don't think a single coin can replace it,
BNB and ETH will also not be as you say, apart from that BNB is one of the potential coins,
let's see the development of BNB in the future I'm curious about that