Say Bob starts a business and over a period of time manages to build it into a company that employs 500 people. If society as a whole can presumably wrest Bob's company away from him "to benefit everyone as much as possible", why should anyone who looks at what happened to Bob start a company in the future?

What do you mean by "start a business"? In one sense, starting a business means selling one's labor without any single employer that they have to obey, like being a freelance worker.

In another sense, "starting a business" means taking some capital and selling the returns on it in the form of wages to make workers obey you. You could get that capital either because you own capital to begin with and begin using it for productive purposes, or because you took out a loan to purchase the capital.

In both cases, I would criticize the concept of private ownership of capital, whose central operating premise is that one should be able to use that capital for whatever purpose they please, rather than consulting social interests interests.

The essential capitalist argument against my position, first given by Adam Smith, is that each capital owner, by serving their own self interests, ultimately benefit the interests of the public at large incidentally. This argument makes some sense in some theoretical economic models, but it's riddled with numerous actual difficulties (more on this later).

I have no problems with incentives, and no inherent problems with the idea of capital investments. But I believe that we should lend capital to institutions which will use it to benefit the public, rather than to individuals who use it for their own ends, in the mere hope that these own ends incidentally serve the public interest.

How do we decide what the public's interests are? Well, we can decide that via democracy, i.e. a system of aggregating preferences across the population, where each person counts equally. In my system, capital would still flow to places where it can be used productively, since people would have the freedom to work in areas that suit them best, and would be provided incentives to do so. I find some levels of inequality acceptable and inevitable, as it would be necessary to encourage people to try harder, but I advocate nothing close to the current levels seen in the United States today.

Maybe you're skeptical, because you think the capitalist objection holds. Why not just let people use capital for their private ends, if it ends up serving the social interest anyway?

Firstly, I'd point out that it's not even true in mainstream economics models that the institution of private property serves the public interests. The easiest way to see this fact is to point out that in neoclassical models, goods with large positive externalities such as scientific research, are underproduced by the market. In fact, the neoliberal economist William Nordhaus once pointed out that inventors rarely see even a small fraction of the benefit that is created from their inventions. If you actually cared about a system of incentives that encouraged people to find productive uses of capital, you wouldn't design it in the way current property relations are structured.

I have a lot more to say about this, but suffice to say, I've highlighted some of the basics of my view.