Taking this long-winded thought experiment back to the point: 99% of her network and her WoT is her family. one family member has a key issue and generates a new nsec. 98% of her WoT says "yea this new key is the same person. looks good". From Grandma's point of view, there's nothing more trustworthy. Basically the entire universe says this is okay. What does she care if 100,000 other npubs not in her WoT don't weigh in on this issue? Why would those 100,000 other npubs form an opinion on this topic? I suspect maybe you're thinking in global terms.