When people say they'll want to defund the police, they are not saying there should be no police. People want a different police. There are structural problems that are not easily fixed, as we have seen. Making a new police-like organisation from the ground up is a way to fix those structural problems. I'm not sure if I agree that is the best solution, but people that do are trying to improve the country that way.
In fact, all people that are protesting do want to improve the world. But they realise that they are held back. See climate change and inequality. Those problems cannot be fixed without getting the people in power to help.