Your question is fair but it follows the same logic as someone saying «people fought with pistols and sabers and stones since beginning of time, so what is the problem in one of them using an atomic bomb instead?».
During my MBA, we had some advanced negotiation courses in which there was lot of tralala as in every MBA but I remember one interesting and important concept: negotiation is in fact a process of information gathering. The more you know about the other side the more you can take advantage of him. But we also learned that we should exit negotiations when information asymetry is too much in favour of the other side and if there is no possibility to rebalance that.
In your example, the tale of «a good family man who sells car in a showroom», the salesman and the potential buyer start the conversation with very limited information about each other and can transparantly ask for more information. For example the salesman could ask «whats your job» or «how many children do you have». And conversaly the buyer can inquire about the car and the brand… This process is balanced and helps creating a trust environment, in turn helping to close deals that are on average fair to both sides. Sometimes a salesman will sell more than needed at a higher price but sometimes smart buyers can also squeeze the margins and get a car at discount. THAT IS FAIR.
With the tools of big data, citizens still have almost zero information or what they receive as an information is in fact carefully selected by producers and advertisers and sent to them in a unidirectional way. But on the other hand sellers have the possibility to knows literally everything about people.
So in your story the citizen is the same, but you replace the salesman by a phd in psychology working for a company like cambridge analytica, potentially trained to conduct military propaganda operations, and knowing everything about you. By all microeconomic and negotiation theories, citizens should run away from any marketplace where information so imbalanced because the whole thing is designed to screw them up and take away any drop of consumer surplus.
No surprise that adblock grows 30% every year. Citizens intuitively feel the process is flawed by design and try to find exit path from this unfair marketplace. People trying to make business on the internet should care a little more about ethics... And first step should be to give citizens back the control of which information about them they want to share and to whom…