Not just that, it's also about weighing the restrictions on personal freedoms against the extent of negative externalities. In some cases the protection of personal freedoms is absolute. The ban on torture is a precedent for that.

>Now, there is actually precedent for banning consenting adults from sharing private information in order to ensure the survival of the democracy [...]

It's a precedent only if it's comparable, which it isn't.

You are overstating your case. There is no reason to believe that letting an algorithm parse some of your data for clues as to what you might want to buy makes democracy fall apart.

What is likely to have an effect on democracy is when governments analyze data for political reasons. Where that happens, people will simply stop volunteering so much private information. They don't need to be banned from doing that.