in 2016-2020 Ethereum can be said to be the king of altcoins, because there are lots of altcoins built on the Ethereum network,
but starting in 2021 Ethereum no longer has a good ecosystem, because currently there is Binance Smart Chain which can be said to be the most successful rival of Ethereum,
yes, because the Gas Fees from BSC are cheap, BSC is currently popular with users, even the ecosystem of BSC has started to grow.
I don't really understand that for what purpose you are actually take the BNB to Ethereum position. If you consider the market volume and the pricing then you will see that Ethereum is much ahead from the BNB. But it true that BNB was one of the best Cryptocurrency who are doing really good fast improvement. But that's doesn't mean that it is take over the Ethereum position. If we talk about the 2021 condition the there will also be seen that volume of Ethereum market is the second position after bitcoin and top in the altcoin. Even then I also think that from the security Ethereum is the best choice after bitcoin.