This question has a really simple answer, Binance smart chain is the straight winner because of the fact that it's really efficient and requires really low transaction fees as compared to Ethereum, whereas for me in ethereum it once required a 29$ gas fee for a 10$ transaction [it may differ for you] which is really high and I won't recommend it for sending and receiving a small amount of assets.
If good scalability on the BSC network will continue to maintain, users will always enjoy the quick and low fees of using it.
But before, the Ethereum network also has the same feature, cheap fees while users can enjoy too a quick transaction. It's no assurance that the BSC network will avoid ending up on that issue too in the future. For now, let us enjoy what network is giving us convenience. In the first place, other networks chains are build to somehow solve some of the issues when using Bitcoin before, especially fees.
BSC for me is a good alternative for ETH while they are still in the process of solving the high fees. Maybe that's the reason why BNB is more popular now, but it's a long way to go as BSC network has not achieved yet the number of projects using ETH network that cause the spike of fees, and maybe you are right, eventually in the long run BSC network will be congested and the fees will increase.