The only thing they can really do is to try to make data hoarding less profitable than it is now. FWIW I think this is a very practical response far more often

The only thing they can really do is to try to make data hoarding less profitable than it is now. FWIW I think this is a very practical response far more often than people realise. "Follow the money" is advice as old as money itself. Making behaviours we don't like unprofitable is one of the most reliable ways we have ever found to guide commercial organisations away from those behaviours. For example it should be not just perfectly legal, but encouraged to scrape and repost bigtech data elsewhere. So that if you're unhappy with Google keeping your data on Google drive, you could switch to another cloud provider and they would already have all the same data. Again FWIW I agree with busting the data hoarding walled gardens and supporting data portability. I couldn't disagree with you more strongly about making it all public by default but I also don't think that's the only way to solve the portability problem.