Zero Data App
As someone dabbling with such an app right now, there are several hurdles if you want to exclusively use self sovereign identity.1. Local data gets lost, so you need to sync for backup. But when your device gets lost or when you need to access it from elsewhere, how would you login to the said backup service? The most intuitive option is still a username and a password.2. It's hard to transfer data from one device to another. And to asynchronously share data with friends and family. Solutions like ipfs exist, but in practice they rely on many centralized services (eg: for pinning). And besides, securely sharing private data is not an option on any of them.3. Lay people being able to safely handle cryptographic keys is an unrealistic assumption.So IMHO, perfect 0-data is not going to happen. It used to work earlier (pre-internet) because we had only one device, didn't  have a lot of media, and could carry floppy disks for the rare occasion.What I think would make a difference is: browser apps being able to do TCP/UDP (with safe-guards) and sandboxed local disk-access. This allows web apps to do things like SSH, and login to the vast storage infrastructure that already exists - such as GitHub. Data remains fully under your control and becomes portable. Git IMHO is a better path to where Solid wants to go.  Especially once SHA256 commit hashes land - to become your personal, consensus-less blockchain.