You don't know how big 2^256 really is. It's so large that a collision if found would take a very long time and would not be found again so soon. For clarity, there are slightly less than 2^256 possible private keys for Monero. 2^256 is "exponentially bigger than the number of atoms in the universe."

Thus you should argue from the point of cost, rather than security. Only researchers would try to find collisions, probably without brute force.