The price of the bitcoin is unstable, a month can go up to 50% but could drop by up to 50%, it is very risky for the corruptor to save money because there is no money opportunities bitcoin runs out.
Funny you consider this as an issue for the corruptor, although this not seems to be a big problem for them. As long as they store and keep their bitcoin no matter how low the price of bitcoin at the moment, they can still wait for it to increase. I think the biggest reason of them using bitcoin is that to hide their corrupted money so that they will not easily get caught.