When people ask for a block-cipher with a larger key space than AES-256 replies usually just state that there is no need. I see where that is coming from the future is hard to predict.
The two main problems I can see are Grover's algorithm (see below) and the fact that we can not see all the problems.
I have no idea how powerful quantum computers will be in 2120 so we may consider upper bounds. e.g. At 20ºC, the Landauer limit* is about 2^-80 watt-hours per bit. I do know that some other ciphers (like threefish) have larger key spaces.
The fact that AES may be broken within 100 years (exactly) was apparently addressed by the OneHundredYearCryptography project by XORing it with XSalsa20.
To reiterate I want to know if AES-256 will be safe throughout our life time.
*To the best of my knowledge, experimental quantum computers that operate at the limit are actually much faster than other experimental computers that are also at/near the limit. I don't know if there is a price difference however.
Edit for background: Grover's algorithm is an attack that is analogous to brute force attack. The differences are that its complexity is the square root of the key space (2^(256/2)) rather than the key space itself, and that it can only be run on a quantum computer. I'm confident that no conventional or quantum computer will be able near-brute-force it, but I'm not that sure that a quantum computer won't be able to Grover it. Therefore, the power of quantum computers by 2120 is of more interest than the power of electrical computers at that time.