I continue to fundamentally disagree with the way you’re framing this issue.
There’s never any obligation. It’s not like the level of rudeness of ghosting someone starts at 0, then stays at 0, and then suddenly goes to 100.
Rather, at least in my opinion, it gradually increases as the relationship grows closer and longer. The more communication you have, the more you are a part of your lives, the greater it grows.
BUT, and here’s the key point where I think we disagree… something being rude doesn’t mean that it’s an obligation. And something not being an obligation, and even being a clearly sensible decision, doesn’t mean it’s not rude.
I’m going to make an analogy here, please recognize that analogies are not perfect equivalences, yada yada yada.
If you’re out at dinner with someone, is it rude to excuse yourself to go to the bathroom, and just slip out the back door and never see them again, right in the middle of the fish dish? I mean, that is OBVIOUSLY rude, right, something no one could possibly disagree with?
But… suppose you get a call while you’re in the bathroom and a friend is letting you know that they have some information which leads them to believe that there is a 15% chance that the person you’re having dinner with is a notorious sexual predator. What do you do?
Well, I don’t know what I would do. But I certainly wouldn’t blame you for just leaving ASAP (assuming this is someone who you’re on a date with, doesn’t know where you live, etc). But it’s still rude. And while you’re deciding what to do, you should factor into your decision the affect your action will have on other people. Other people in this case presumably being your dinner companion. Because that’s what decent human beings do. They consider how their actions affect others. BUT, and here is the keyest point, CONSIDERING how your actions will affect others doesn’t mean that that factor has to be the deciding one. It’s entirely reasonable to make a subconscious calculation like “well, 85% chance he’s not a sex predator… in which case me ditching him is rude, will cause him some pain. But 15% chance he is a sex predator, sorry, dude, that’s just too much. Gotta ditch you.”
You should consider the effect your actions will have on other people because that’s an insanely basic facet of every system of human morality. If an action you take is likely to cause someone some amount of unhappiness, that’s a thing you should consider, something you should weigh in. And if you weight it, and decide that other factors, such as your own personal safety or convenience or sanity are more important, that’s utterly fine.