> Data portability is a huge value prop: Everywhere you sign-in, you already have all of your data.

No. No you don't.

1. Below you claim that "Only you (or anyone who has access to your private key) can access your data."

So when you sign in somewhere, that "web3" app still has no access to your data

2. Just the fact that you have your data does not mean it's portable.

A proprietary binary blob created by app1 will not be readable or accessible through app2.