Yes, a useful last resort safeguard would need to be more granular than just "turn everything off", at least if we're talking about protecting production systems rather than just people learning and wanting to avoid inadvertently leaving the free tier or something like that.
Still, it's not hard to imagine some simple refinements that might be practical. For example, an option to preserve data in DB or other storage services but stop all other activity might be a useful safety net for a lot of AWS customers. It wouldn't cut costs to 0, but that's not necessarily the optimal outcome for customers running in production anyway. It would probably mitigate the greatest risks of runaway costs, though.
Still, it's not hard to imagine some simple refinements that might be practical. For example, an option to preserve data in DB or other storage services but stop all other activity might be a useful safety net for a lot of AWS customers. It wouldn't cut costs to 0, but that's not necessarily the optimal outcome for customers running in production anyway. It would probably mitigate the greatest risks of runaway costs, though.