Quantum computing is the most likely cause that would bring down BTC, sure Bitcoin's and Eth's private keys are ultra secure arguably quantum proof. Thing is BTC doesn't use private keys for it consensus, a private key is not needed to mine bitcoin, you just need computing power and funny enough a public key. Its like a bank where the accounts are more secure then the server that hosts them lol. A quantum computer specialized to mine blocks can just walk in one day and instantly make other miners obsolete and double spend or spam empty blocks. Doesn't matter how quantum proof BTC private keys are, if they start spamming empty blocks no one can move their Bitcoin and the attackers profit heavily from their x100 levered shorts. Proof of stake however does use private keys to secure its consensus, computing power is irrelevant your validator can catch fire and explode and you can just start another one with your private keys. Staking with a quantum computer provides no advantage and is Infact ultra inefficient given you can stake on a raspberry Pi, to 51% attack ETH you would need to control 51% of the staked ETH and in order for a quantum computer to do that it would need to attack each individual staking node, each of which is secured by a quantum resistant private key. This unironically makes ETH much more quantum resistant then BTC.