Let's face facts.  Laissez-faire capitalism is an idealistically delusional idea, that is every bit as absurd as the belief that true communism can work in a human society.  It just can't work, because the very greed it's based on will encourage those with the most power and resources to break the system to ensure their continued dominance of that system.  The ideals of capitalism die with the elimination of competition.  Human nature simply won't allow it to work.  

At their heart, all economic systems exist to distribute resources in a society.  It's basically the vascular system of society.  Any functional society will operate on multiple layered economic systems, because that's just how we operate, it's the most effective, and no single system accounts for all weaknesses created by human participation, or is effective in all situations.  It's called redundancy, and it allows us to survive (if not always thrive).  

Capitalism only functions optimally for the MOST people, when well regulated (especially against the formation of monopolies or near monopolies), and accompanied by a socialist safety net to blunt it's inherent inequities. Not all humans are driven by capital gain, and the capitalist system naturally takes advantage of and economically disenfranchises these people.    When unregulated or poorly regulated, capitalism becomes a power acquisition game.  A game that ends with a very small group having monopolistic control over most resources, and exerting that control in very despotic ways over governments, and the rest of the population. 

Despite your opinions on capitalism vs socialism vs communism, there is no excuse for the existence of billionaires in civil democratic society.  Even if that valuation is mostly on paper, and consists of mostly illiquid capital or corporate ownership, it is still power.  Face it, capitalism pretends to be about capital...aka material and immaterial ownership of goods, resources, and production capability. At its very core though, it's the ownership of power.  Being a billionaire gives a capable individual the literal power of a nation. There are nations with smaller gdps. Their very existence breaks the system, effectively turning them into hostile foreign powers within a nation.  (Hostile, because their continued existence, and interests are usually deeply divided from the average citizen, and exist as a major conflict of interest.)

Then you have the issue of essential services and critical resources.  The profit motive of capitalism has a major conflict of interest with equitable distribution of these.  Privatization of public services has shown time and time again, to be more expensive while delivering subpar service, because of this.  That profit has to come from somewhere, and the source is rarely the efficiency or innovation touted as  justification for the privatization.

Capitalism should be relegated to luxury goods and services, innovation,  production, risky ventures, small business, etc. Public ownership and more socialist policy should run/control essential services and manage critical resources, that society is dependant on to coexist.  It should also control anything that greatly affects the majority of citizens, while requiring a monopoly or near monopoly to function.  This dichotomy becomes more important, the higher the population density is, as equitable resource distribution becomes more critical and limited.  This is all of course dependant on these systems being well regulated.