There's a subtle difference between what each of us is describing I think. It seems you're arguing that wearing headphones in public isn't generally inherently rude, and you have a selection of anecdotes to support it, and I'd agree that those are plausible and fine.
Incidentally I have no interest in living in SF, Boston, or Japan for various reasons, but it is interesting that I also wouldn't necessarily anticipate they'd be suitable for completely random friendly, welcomed interactions, in transit or wherever. Japan gives me a very siloed, weirdly socially isolated vibe, Boston I haven't been to, and SF just gives me a sort of aggressive individualist capitalism achievement chasing vibe. California cities in general feel like a departure from what I like.
However, I'm not exclusively talking about literally accepting every random encounter in public transit or on the street only. In those places, you do have bars, gyms, third-spaces, cafes, etc.. that are for the most part, "public"; you should be able to connect with people in your community and avoid signalling that you aren't there to participate.
I'd basically agree that wearing headphones out in the world at all is not inherently rude, but wearing them in all of those places, an overwhelming majority of the time, intending to avoid almost any interaction with strangers, is deliberately socially rusty. It almost seems like a strawman I'm arguing against, but it's my impression that's what the ease of AirPods enable, compared to larger less-convenient headphones. You can leave your house with the AirPods in, no wires, take calls without your hands, go to the gym, do your shopping, go to the park, go for a jog, bike ride, and keep the world out end-to-end. It's my view that unless you have a real sensory issue, that's shit for everyone involved at some scale.
Edit: I'd accept that I may not have articulated that so well in prior comments, but thought the overall was implied to mean the degree to which AirPods uniquely enable this