It certainly seems like in the future there's going to be a market for user-controlled personal-data stores (using a standard format for certain types of data)

It certainly seems like in the future there's going to be a market for user-controlled personal-data stores (using a standard format for certain types of data) + library glue to load them into apps on-demand, as that'll be the only way to accomplish features like this in a privacy-preserving way. Or, you know, the platform companies end up being the only ones who can access the on-device stores from local apps.