I see that comment all the time in comments on EV articles, and it's almost always accompanied by conspiracy theory nonsense about how "THEY" won't allow that to happen, because it would prevent "THEM" from charging people money for recharging.
So a couple of ideas for answering this.
People get this idea because they think that generators just spin freely and generate energy without taking force. So they see something spinning, like the wheels of the car, and think they can add however many generators of whatever size they want and just collect free energy from it, without affecting the movement of the wheels or requiring any extra input energy.
But suggest this idea. Have you ever been to one of those hands-on science museums? Have you been to the display where they have a stationary bike with a generator on it, so you can see how energy is generated from force? You get on the bike, and pedal, and it lights up a light bulb. But then as you connect a 2nd bulb and a 3rd bulb, it gets harder and harder to pedal because there is more load, and you're having to put in more force to generate more energy output.
Another thing is to remind people that generators and motors are strictly energy CONVERSION devices. They don't produce energy out of nothing. It is taking rotational kinetic energy and converting that amount (minus losses) into electrical energy. So any electricity you want to collect out must come from EXTRA rotational kinetic energy put in! The other part is to remind them that this is a closed loop system. It is having to take extra energy from the battery, to convert through the motor into kinetic energy, to then convert back through a generator into electricity, to then put it back into the battery. It's not gaining anything from anywhere. It's just doing two conversion steps in a circle. And in each conversion, nothing is perfectly efficient, so at 90% efficiency, you're taking 1 unit of energy, converting it and getting 0.9, and then converting it again and getting 0.81. Great--you just wasted energy while gaining nothing.