1) Your equations don't take into account reversible computing advancements at all. These techniques could halve the search space by being able to take advantage of algorithms that traditional computers find impossible.

2) Your equations assume the algorithm is perfectly secure. Given that NIST is rather gung ho about developing SHA-3, I'm not terribly confident in that analysis.

3) An attacker does not need to find a specific key to compromise the system, they just need to find the keys to someone's wallet. If you want this to be a viable currency for Earth's population, plus corporate accounts, you drop the search space by 10^10 to 10^12.

That 2^256 search space could get whittled down to the 2^110 range by the end of the century. Assuming no major attacks on its integrity exist.

I'm not particularly sold on the technical soundness of this program, honestly. Why use SHA256 rather than Whirlpool or SHA512?