I am not sure you can legitimately characterise the efforts of an intelligent agent as being "random stumbling".

Anyway, I was pointing out a flaw in the reasoning supporting a small probability of failure (under the described circumstances). Maybe some other argument supports a small probability of failure. However, the original argument would still be wrong.

Other approaches - including messy ones like neural networks - might result in a stable self-improving system with a desirable goal, apart from trying to develop a deterministic self-improving system that has a stable goal from the beginning.

A good job too. After all, those are our current circumstances. Complex messy systems like Google and hedge funds are growing towards machine intelligence - while trying to preserve what they value in the process.