The new thing, and the key point here is scale.
You can discuss persuasion or cognitive bias away until everyone gets an aneurysm, sure. Some example: You can't not communicate. So even not persuading is persuading. What if the presidential candidate would just not give a scheduled speech? How do you communicate information objectively? Casual language creates bias, so does scientific language, simple language, passive language, active language. "Cognitive Bias" might as well be called "Cognition", since it is just how the brain works. You have to think "tree" immediately when you see one, even before you validated that all leaves are real and that the whole thing is not a projection on an transparent screen. Otherwise you can't function.
But: Big tech throws us in a situation where a small group of people influences our perception on a massive scale. Facebook changes a sentence on their homepage and a billion people read it. Youtube raises some parameter (yeah I know that's not how AI works) by 0.01 and the political opinion about the Grenfell tower disaster changes ever so slightly - for 30 million people. Google's filter has a tiny hole and some troll broadcasts wrong medical information about gout to 200k people.
Every time one of these things happen, the world shakes. Dozens die or survive. Demonstrations form and elections swing. Opportunities are wasted and ideas surface.
I am not arrogant to actually propose an easy solution to this, and I don't think there is one. Just be aware that "I can always go and stab someone" is not a good argument when you are discussing a fully automated drone swarm with kill authority.