What’s a fundamental non-currency infrastructure affordance of blockchains that can’t be delivered with web2 or older stacks easily? 

A near-example is NFTs bought on one site showing up on another, which you can’t do in web2 (closest is walled garden/game badges). 

Things like this but more infrastructurey in flavor, and neither finance related, nor fun and games. Maybe like DeFi for nonFi things?

Delver at contraptions.venkateshrao.com, running summerofprotocols.com, host of /gloom ☀️

You can't offer credibly neutral PKIM where I can guarantee the only thing that can update my entry in the database is my private key.

This is the design for the Farcaster registry. Every web2 version has some server you could rip out of the wall.