4.7M of the 8M US businesses have <5 employees.
https://www.census.gov/newsroom/stories/small-business-week....
Starting a business requires little more than filing incorporation documents with your state.
Growing a business to dominate a market and get crazy rich may require outside funding, but growing a business to support and maintain your individual lifestyle does not.
On the other hand, it makes it easy for your competitors to reach customers.
I'd argue that, on the whole, the internet has made starting "just an idea and a garage" businesses harder, because they now face immediate, maximally-funded competition. Whereas pre-Internet they would have been geographically/physically protected for awhile.
True, net win for customers, efficiency, etc. (maybe). But you couldn't start a Starbucks these days.
Their income is similar to that of most working professionals and their assets are similar to the savings of most working professionals. They entrepreneured themselves into having a job, they're not the investor/capitalist multimillionaires we picture when we think of rich people.
>the world would exist as a series of 3-5 megacorps, and small businesses would not exist
The economy exists as the 500 megacorps that make up the s&p 500. Small businesses just fill in the gaps and are for the most part beholden to the megacorps for their business, their supplies and/or their financing.
If what you say were true, the world would exist as a series of 3-5 megacorps, and small businesses would not exist. Since they do exist, how do you square that with your claim that only the "extremely rich would be able to start a business of any size" is currently true?