Liberating user data from the platform monopolies?

Dear Listers,

Just a random thought. What if the law entitled users to a periodic (eg.
quarterly) download of all data collected of them by large platform
companies (including telcos); and the users could grant file access to the
competitors as the user migrates? What if it was API-based data access that
the user could grant to third parties including competitors?

What if there was an automated data depository eg. for telcos; whereby I
can invoke the right to access all the data the telco holds of me, and I
proceed to grant the data depository access to my data, automatically
retrieved from my telco A. That way my data *anonymized or otherwise
permitted* can be accessible from the depository and aggregated with other
peoples\’ to develop services on top of the data layer by third parties such
as startups, data mining & research services, and even telco B which
competes as an underdog with telco A.

The assumption would be that the user owns the data held by the
platform/telco which has already got its head start with appropriating my
data by collecting it in realtime. It would also be assumed that access to
my data in the depository is only by licensed entities under stringent
rules that respect my access permission settings.

What could go wrong with such an approach to user data? What could go well?

Any thoughts?