Market forces only work when no individual or even large groups of individuals control enough capital to influence market trends, the problem is because of the

Market forces only work when no individual or even large groups of individuals control enough capital to influence market trends, the problem is because of the dismantling of capital gains taxes etc. we have exactly that. We live in a state of affairs where individuals own over 80% of rental properties in entire suburbs. They can then do whatever they like with rent, since there is no such thing as market forces in those areas. They then collude with similar individuals in surrounding areas and create an unregulated monopoly. How do you propose that is beneficial?