> One of the primary flaws of capitalism is that there is little incentive to enter a lower margin business when their is still potential profit to be made in a higher margin business
That's quite easily disproven throughout the modern history of market economies. For one relevant large example, low and high margin retail rapidly expanded and evolved simultaneously throughout the US market during the 19th and 20th centuries. Dollar - nickel & dime - stores didn't wait until all the high margin business potential was gone.
Sam Walton didn't decide to go high margin just because there was still some obvious growth left in Macy's business in the 1960s. He saw an opportunity elsewhere.
There is tremendous value in getting there before someone else does, rather than wading into extreme competition just because a segment is higher margin (which does you no good if you die in the fierce competition). Low margin can be a spectacular business thanks to volume. Just ask Walmart, as a low margin retailer they've generated more net income over time than any other retailer that has existed.
Another counter to your premise, is that if you build a successful, high volume, low margin business, it is extraordinarily difficult to compete with. And you can easily end up earning as much as a high margin retailer with lower volume. Walmart and Amazon understand this very well, as do dollar stores.
The myth of profit maximization is perhaps the greatest myth having to do with Capitalism. I'd argue that in fact few businesses pursue max profit, much less profit at all costs. It's a caricature of S&P 500 type companies. It goes along with the commonly espoused myth that there is a legal responsibility to shareholders to maximize profit. Most businesses that stay in business understand there are dangerous, very negative consequences to pursuing max profit, as it pertains to customers, employees, reinvestment, etc.
Your premise is strict on theory, weak on actually being how business founders & operators think and how / why they start a business in the first place. The typical business founder doesn't run a massive analysis of the US market city to city, for years on end at great cost, and decide there's a bit of profit left in high margin, therefore they won't enter low margin. That isn't how a business founder thinks at all. The actual chaos involved is breathtaking, it's high on impulse, gut feelings, 'instinct,' intuition, a sense of opportunity, and low on following some kind of rigid theory and massive market analysis.