See, this last conclusion of yours is a fallacy. All you (strictly) need to prevent this (very hypothetical) scenario you described is either enough admins so that there's always someone around to stop such a bot quickly enough, or a cap on the maximum number of blocks (i.e. desysoppings) that any one user can do in a fixed amount of time (let's say, one per hour).
You can't block an adminbot, so we're talking about desysoppings, not blocks. If anyone can desysop anyone else, then you'll get malicious desysoppings, which would need to be undone, either by some higher rank, so by allowing everyone to make sysops. If everyone can make sysops, then a vandal with a handful of sockpuppets can do constant vandalism, always resysoping any of his accounts that are desysopped.
If you code a cap on desysoppings, the vandals simply have to use sockpuppets. If you code a global cap on desysoppings by any account, then simply having one more vandalbot than that cap makes you untouchable.
Whatever ideas you come up with, in order for them to work, you end up having a higher rank. You can keep proposing ideas if you like, and I'll keep telling you where the higher rank is required. I'm confident there will always be one.