Earlier quoted context omitted.
Right, the fact that they are successful in Capitalism presents the possibility of building the entire economy around them. It would be a free market of independent businesses, but those businesses would be governed by the workers involved in them. Financing would happen through loans and crowdfunding, rather than equity. This economic system wouldn't be Capitalism, but it would be a Free Market. It would involve "so…
What would be the advantage of removing sole proprietorships, partnerships etc though? If worker coops are already one existing type of business in a capitalist system, why would we limit ourselves to just that type and remove others?
On top of that, work is where most people spend most of their lives. Right now, most people work at businesses that are governed in an autocratic way. Which means, on a very practical level, we're asking most people to spend most of their lives operating in an autocratic context.
With respect to Sole Proprietorships and Partnerships. There are some pretty big sole proprietorships and partnerships out there - with lots of employees. The bigger they get, the smaller the contribution of the sole owner or partners to the business vs the rest of the employees and the more the current system gives an unjust distribution of rewards.
Sole proprietorships and partnerships wouldn't vanish entirely. They'd just be limited to businesses where the sole owner or partner really could do all the work (or most of it with some of it maybe contracted out to other cooperatives).
It's true that the Free Market Socialist system described above probably would undervalue the contributions of founders. But the current system massively undervalues the contribution of the average worker. So it's a net gain in terms of justly and accurately valuing contributions. And Free Market Socialism would result in a much more even distribution of wealth, and therefor power.
It wouldn't be perfect, but - personally - I think it would be an improvement.