Regarding #1, owners operate a factory that sells goods to the masses so they can get the labor or products of the labor of the masses (indirectly by getting currency they then use to buy that labor)
If with enough automation that's not necessary then it doesn't matter. Why operate a sneaker factory to get the money to buy caviar from the caviar factory when you can just operate the caviar factory directly?
Put another way, china thanks to it's new middle class is a great market for a business to sell to. That's because they have something of value to trade back.
But the Congo, full of people or not, isn't. They don't have anything any capitalist could want that would equal the value of finished middle class goods. All they have to offer is cheap raw materials or perhaps labor to extract. If people are made useless, we will all be from the Congo except the legacy capital owners, who can trade with each other for everything they might want.
Regarding #2, they would form their own garbage shit economy whose size and per capital size are like specks of dust compared to the main one, with obvious consequences for power dynamics when the two economies interact. Think of the underground economy in a prison, a homeless encampment, or a warzone, or subsistence farming peasanta deep, deep in thr Congo. Or the "economy" formed by insects trading pollen services for nectar with plants. That's how small and weak the economy of the leftover people with no access to the global industrial machine would be by comparison.
1. Supply drives demand. If nobody produces anything with which to buy the factory output, then the factories will not be valuable.
2. If most people are somehow living "outside" the economy, then they would form their own economy.
Those are but two trivial rebuttals, and by no means anywhere near exhaustive.