That depends what your initial probability is and why. If it already low due to updates on predictions about the system, then updating on "unpredictable" will increase the probability by lowering the strength of those predictions. Since destruction of humanity is rather important, even if the existential AI risk scenario is of low probability it matters exactly how low.

The importance should not weight upon our estimation, unless you proclaim that I should succumb to a bias. Furthermore, it is the destruction of the mankind that is the prediction being made here. Via multitude of assumptions, the most dubious one being that the system will have real-world, physical goal. Number of paperclips is not easy.

On further thought, this is not even necessarily true. The solution space and the model will have to be pre-cut by someone (presumably human engineers) who doesn't know where the solution actually is. A self-improving system will have to expand both if the solution is outside them in order to find it. A system that can reach a solution even when initially over-constrained is more useful than the one that can't, and so someone will build it.

Sorry, you are factually wrong as of how the design of automatic tools work. Rest of your argument presses too hard to recruit multitude of importance related biases and cognitive fallacies that were described on this very site.

If you have a trillion optimization systems on a planet running at the same time you have to be really sure that nothing can't go wrong.

No I don't, if the systems that work right took all the low hanging fruit from picking by one that goes wrong.

Well, I in turn believe you are applying overzealous anti-anthropomorphization. Which is normally a perfectly good heuristic when dealing with software, but the fact is human intelligence is the only thing in "intelligence" reference class we have, and although AI will almost certainly be different they will not necessarily be different in every possible way. Especially considering the possibility of AI that are either directly base on human-like architecture or even are designed to directly interact with humans, which requires having at least some human-compatible models and behaviours.

You seem to keep forgetting of all the software that is fundamentally different from human mind, but solves the problems very well. The issue reads like a belief in extreme superiority of man over machine, except it is a superiority of anthropomorphized software over all other software.