This is a reminder that if you're not the client, you're the product.
Much of the time, you are both the client and the product. See, for example: Cable television.
Saying that a true statement is "trite" and "meaningless" doesn't make it untrue.
Just because you are a customer does not mean you won't be monetized.
While it's true you can be paying someone to sell you to some third-party, it's virtually never the case where a private firm offers you a service completely gratis without sell you to some third-party. Furthermore, without the exchange of funds, you lose a lot of legal protections and the ability to hold the company accountable for a lot of their actions.
I know it might be annoying to hear it over and over, but I think it bares repeating because I can guarantee you that only some people have heard that phrase, and far fewer have truly understood it's impact.
Further, it is cited as a rule but it is far from one. Paying for a product doesn't guarantee your privacy and using a free product doesn't mean your privacy is being violated.
For ex. you can pay for Google Apps over Gmail but the privacy policy and terms of use is still the same (I actually can't think of a single service with a pro paid level where you gain privacy - Flickr, Dropbox, Freshbooks, Mailchimp, LinkedIn, Salesforce, Office online - you name it). Likewise there are countless examples of free and open source software that do respect your privacy.
I don't think there is a shortcut to teaching the general public about privacy - especially not one that can be wrapped in a one line cliche.
While what you say is logical, it gets scary pretty quickly. What about uber? What about airbnb? Or worse, what about paypal, intuit, wells fargo?
I propose a simple thought experiment as a band-aid. If we can't beat them, we must join them. Every company that collects information about me, must disclose the said information to me. Failure to disclose in a reasonable time frame should result in an automatic fine worth 100x minimum wage per hour every hour after the end of the reasonable time frame. Of course, we'd need very strong whistle blower protection. This would be a terrible idea because if it works (and I doubt it), it will have a huge chilling effect on small businesses. Responding to all the requests would put them out of business.
I don't know what the solution could be... but I know educating people is difficult especially when the people don't want to be educated.
At some point so many people started annoying Facebook with these requests that they added a self-service option to download all the data they have on you somewhere in setting. (Google has something similar, "Google takeout")