If the adoption of Binance Smart Chain (BSC) is growing compared to ETH, this would be a sign of a threat to ETH. Moreover, for now BSC offers lower fees than ETH smart contracts, so that, it makes new coins more interested in using the BSC platform compared to Ethereum (ETH). But in terms of implementation, BSC still needs greater community and developer support to shift ETH, because the one of the shortcomings of the BSC is regarding control which is still held by Binance itself, whereas ETH is more decentralized.
I would say that they could grow bigger together as well, I do not think that it will be impossible for one of them to win and other to go down but I still think that the competition between them could probably result with the price of both of them growing bigger and bigger together. This is why I honestly believe that there is a good chance we could end up with something profitable for both ethereum investors and bnb investors as well. This could spill out to people who end up spending money on both cake and also uniswap as well.
I know it is not easy to handle all of that but I think it is quite possible and if you divide your investments to all of them that should be enough for you to make a good return. There are also lesser ones, like sushi and apeswap which are probably secondary or what not compared to high end ones but if all the others go up then these could make you even more money.