BNB is not threatening ETH only because of ETH's high fees; although it helps.
Binance, the biggest crypto exchange in the world, is offering incentives for users to stop using ETH and start using BNB. that's a lot for ETH to compete with. Imagine what an altcoin project would pay for 24/7 advertising on every page of Binance. BSC gets this for free.
If you haven't noticed, DeFi/NFT users don't give a shit about decentralization. They only care that it's open and without KYC. CZ is taking advantage of that fact by offering a more centralized network, but the trade-off is that it's 100x more scaleable, cheaper and faster. A long criticism of ETH is: "If this is useful and successful, then competitors will simply fork it and offer it cheaper and faster" (ETH HAS NO PROPERTIES WHICH ARE IRREPRODUCIBLE). Oh, that's exactly what CZ did!
ETH devs have failed to provide a solution for scaling in 5 years. They've simply failed and now they need to deal with more competent competitors. If ETH fees go to $1 tomorrow, they'd still have a very significant challenge.
ETH fees are more likely to decrease because people stop using it than ETH devs do something innovative for scaling.
ETH 2.0 promising a solution for this, and people waiting for it. But, I'm afraid if this just a hope, because in reality too much delay from dev. In other side, BSC look like easy to change their token tx fee. Few weeks ago we need to pay 10 GWEI for transfer BSC tokens, and now it reduced to 5 GWEI. From this, I'm thinking about why BSC can do simple things like this, while ETH need to change entire network to fix current gas price issue.