there are economic incentives to help others both weak and strong.
There are not, unless your include the other factors like morals in the usability function of people. Even if you do include those factors, no study has ever shown that it was enough.
Let me be clear: I'm not saying that anarchy cannot work because of this, I'm somewhat of an anarchist myself. I'm sure there are lots of solutions. Yet this is an issue, as you cannot rely on a free market to fulfill the needs of everyone; nowhere in the definition of a free market can you see that there is some guarantee that everyone can achieve survival. Dismissing the issue is just wishful thinking.
You are not responsible for fulfilling everyones needs. I am not dismissing anything. I'm telling you that I see needs being satisfied right now voluntarily. I also see an organisation that prevents people from meeting their needs every day. I am not saying that the second people stop interfering all needs will be met, only that the process of meeting needs is inhibited by the use of force, not helped by it.