In practice, they currently can. Tech companies have a monopoly in their respective verticals due to the lack of interoperability and use network effects to essentially force you to submit to whatever terms they choose.

Keep in mind that regulation isn’t usually drafted in a vacuum and instead takes the real world into account.

If tech monopolies get broken up by anti-trust regulation it would be a good time to review the GDPR (as privacy-friendly competitors can now interoperate with existing social media networks) but until then I’m happy to have it.