BNB's use is limited to Binance exchange and its DEX. Binance in some time in history if failed to keep up with the growth, than the price of BNB would decrease. With a large number of projects building new generation DEX and CEX, the time of binance are limited. But ETH has multiple uses. Even if some other competitor gives in a competition on new market growth, the current market would be enough for ETH to grow definitely.