All of these 'Ethereum-killers' have been a joke so far in attempts to take over the defi market. But there is one thing that could threaten it that I can think about...
BTC does support a very primitive, but limited ability to run smart contracts. It's really not worth the effort though.
HOWEVER... if there was a proper smart-contract fork for BTC, that would definitely cause quite a stir. You'd now be able to run a whole DeFi system on the BTC chain... it would have a larger market cap than just Ethereum, and.... you could make the EVM totally compatible if you wanted so all Dapps would run on it.
But... there are too many BTC lazer-eyed maxis out there, that this would be hard to implement without push-back by the zealots. And there is an irony in that... since it may very well be their downfall.
I am not so sure how many decades BTC can last as a blue-chip when it's only got brand-name as an advantage. When people lose interest in that... there is nothing left because of its simplicity. The first-mover advantage won't be worth any goodwill anymore.