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. 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". 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.