However, in virtual spaces we have no mechanism for such accidents. Once we block or mute others, there is never an incentive to revisit these choices.

Also, I think I disagree with this one: for example, suppose that I block person A, and we both happen to be friends with B. Now if I'm having (or even just reading) a conversation in a virtual space where A and B are both talking, it will quickly become obvious to me that I'm missing half of the conversation, likely because I've blocked someone in the conversation. And then there will be an incentive for me to unblock them, so that the conversation would make more sense.

(This seems to me to be the reason why people so rarely put others totally on ignore on IRC, even though it would be possible in principle: because conversations so quickly become incomprehensible. Also it's awkward and breaks some social norms to just openly ignore someone who's "present" and participating in the conversation.)

There's also the possibility of running into someone on another service, where you hadn't yet blocked them.

I do agree with your overall thesis though, that in general online spaces are much more intentional and physical spaces are much more accidental. And I find it an interesting one, which is why I bother to nitpick. :-)