Currently, as far as I have understood, there is no technical solution around, that prevents app-providers from caching, storing or even selling once POD-released user-data the same way they currently do. Right?
So, currently Solid-Apps will just relieve end-users from having to type in the same profile-data over and over again and will provide a mechanism to some kind of white-spiritual-holy-boly-apps that will promise, that they will never ever store, transfer or even sell (how evil) POD data to third parties (keep fingers crossed).
So, what can we really do to prevent our data going clear-texted into some not so clerical datasilos?
To really prevent being locked into the mercy of the all-dominating “Fearsome Five”, a new generation of Solid-apps must be developed, that all must comply with a legally prosecutable agreement to copy and store POD-data on their side only the way it is intended by their POD owners.
Legal could even enforce that app-side cached or stored data must regularly be counterchecked (similar to Web-Cookies) with POD data for accuracy and accessible that might have changed in the mean-time. So, those apps will have to operate with POD data that will always come with a POD-user defined lease date and legal authorities will have to sack transactions with expired leases.
Another category of apps might comply to a standard, where data is never given away from the POD in clear-text, but in the forms of encrypted tokens that can only be displayed in some kind of standardized mini-viewer-components, that will display the data in a non printable, not copy/pastable way and never as a whole (Imagine an operator that has to actively click every single piece of information that will get the data from the POD and will display it only as long as the mouse is not moved nor clicked. You move the mouse - data is physically gone! We could even go so far, that such components will display realtime-decrypted data just through a tiny pinhole below the mouse - no way to copy all the context.
A different approach would be to underly displayed clear-text with micro-codes that could be - traced in screen-shots, will confuse OCR-readers or can be tracked another way similar to nano-particles in the finish for cars.