This unlimited concentration of capital actually is a characteristic of socialism, and is incompatible with the gains of the division of labor in a free society. In a free society 100% control of capital by a few is against the self interest of everyone. It is against the self interest of the few owners of capital because they will have to self sacrifice and use force to prevent competition. It is impossible for them to know everything, and so there will always be opportunities for new competition. At some point is will be less costly for them to allow some new competition rather than try to completely control everything and wipe out every other competitor. It will just not be worth the time and trouble.
It's not about the concentration of capital right now. It's about the concentration of wealth and income and I don't see how that is a characteristic of socialism. In countries way more socialist than ours income and wealth is not nearly as concentrated as it is in today's United States.