It's not about the access, it's about the completeness. Imagine this paradigm takes off (I hope it does!), everyone has their own PDS and finally owns their data. Social apps link into their PDS to publish and share data exactly as they're supposed to.

Well now someone's PDS is a truly complete record of their social activity neatly organized for anyone that's interested. It's not a security issue, after all the data was still public before, but the barrier to entry is now zero. It's so low that you can just go to stalker.io, put in their handle, and it will analyze their profile and will print out a scary accurate timeline of their activity and location leveraging AI's geoguesser skill.