That seems like a stronger mandate, but I think what I proposed would actually reign in their power better than your proposed decoupling.
First, different services would still have to agree on data formats to interoperate. The ecosystem would be quite similar to windows file formats, and we've got plenty of experience seeing how that works out. It's better than what we have now, but not a panacea.
The bigger issue is that in addition to wielding data lock in, these companies wield network lock in. With storage decoupling, if I load up my own profile in a hypothetical "Facebook competitor" app and make a post, Facebook has no desire to display that post to users of its app that are my friends. However with my API proposal, competitor apps have the ability to publish directly on Facebook's site and be treated exactly the same as every other post.
Also, my proposal has a longstanding philosophical grounding that the abilities of computing should be available to all - companies shouldn't be able to insist on specific methods of usage that computationally disenfranchise users.
Strict enforcement policy on cold storage data warehouses. And sensible policy for the app providers, like not caching PII.
Implement and enforce at the fed level using this division in a manner that is something akin to Glass-Steagall.
Owning data should not be something providers can ever exploit.
[edit]: I also think this could be used as a means to break apart Big Tech