Ethereum was also a cheap alternative to bitcoin a few years ago, but now it is sometimes more costly to use the ethereum network than bitcoin blockchain to transact. I for ine believe that BNB is not that popular even now and sometime in the future when it get enough popular to compete with ETH then also there would a problem arise related to high transaction fees on BNB and then people will again look for a cheap alternative just like now.
Well, we don't know, but what is clear is that the current ethereum fee is really very high, even many times the bitcoin fee. even when you say that current BSC is not as popular as ethereum, but nowadays almost all coins develop chains for BSC because of low fees, even new coins nowadays mostly developed in BSC chain. we can think about it later, because people are always looking for alternatives, but for now, even I think the majority vote, people tend to use BSC over ethereum.