This seems like one of those things that sounds nice, but isn't special enough to supplant the existing good-enough-for-most-people methods that are already mus

This seems like one of those things that sounds nice, but isn't special enough to supplant the existing good-enough-for-most-people methods that are already must more entrenched (phone #, email, in-app QR codes come to mind first). Especially since there isn't a single "online identity" for most people in the US, and as you mention sharing selectively is arguably a feature, not a bug. There are lots of examples of "good enough" stuff preventing adoption of newer tech across the world. I see stuff like the weird US checking account clearance system in this bucket - it would be really expensive to replace, so despite there being newer, better systems elsewhere, it's seen as "good enough" - or the relatively slow uptake of electronic payment in Japan even as cash-based vending machines were far more ubiquitous than elsewhere. That's a lot of infrastructure to upgrade, so advantages have to be extreme, not just present.