is not that hard imho, you if have the right amount of money, you can perform a 51% attack, just be very rich
What amount of money is "right" for you?
And btw how 51% attack works?
more than billions of dollars and 51% percent attack doesn't kill bitcoin anyway.. all it does is allow the attacker to double spend their own coins and disrupt transactions while they have control.
theoretically someone with a quantum computer could break the cryptography but if they did that they would also be able to break into any computer system and any financial institution they want and wreak havoc on the entire global banking system.
here is an interesting article and video on the power of quantum computing to break encryption.