Why do you think other moral systems are more resilient?

I think a mix of moral systems is more resilient. Some consequentialism, some deontology, some gut feeling.

Islamism, unlike the first two, seems to be deontological, like Christianity. Isn't it?

No, I don't think so. Mainstream Islam is deontological, but fundamentalist movements, just like in Christianity, shift to less deontology and more utilitarianism (of course, with a very particular notion of "utility").

although deontology isn't totally invulnerable to PM either

Yes, deontology is corruptible as well, but one of the reasons it's more robust is that it's simpler. To be a consequentialist you first need the ability to figure out the consequences and that's a complicated and error-prone process, vulnerable to attack. To be a deontologist you don't need to figure out anything except which rule to apply.

To corrupt a consequentialist it might be sufficient to mess with his estimation of probabilities. To corrupt a deontologist you need to replace at least some of his rules. Maybe if you find a pair of contradictory rules you could get somewhere by changing which to apply when, but in practice this doesn't seem to be a promising attack vector.

And yes, I'm not arguing that this is a sufficient reason to avoid being a consequentialist. But, as you say, it's a good reason to be more wary.