That's still a _lot_ of freakin' restaurants my guy. Costco is the only counterexample that I know of to "all workplaces need unions in order to guarantee workers' quality of life", but I think they might be the exception that proves the rule. That they've grown to be that big and treated their employees that well is a testament to how good economically, long term, it is to treat your employees as the valuable assets they are.
I also trust Costco a lot more than Amazon, even though Costco is not as convenient to use due to their smaller number of SKUs.
The idea is that if you see something that interests you, you buy it now, and there's always something new on each visit. It's annoying for certain things, but at the same time it allows them to avoid having the same stock on the floor because people are on the fence about a purchase.
So pretty freaking big?
Personally I spend most of my money at Costco when I can because I prefer giving money to companies that treat their workers better (or at least better than Amazon).
The "rising tide lifts all boats" reaganomics argument is not wrong, but if it reaches the point (where it has now) where the working class is not seeing an inflation-adjusted increase in wealth, then they are (by definition) becoming poorer over time.
The issue isn't capitalism its the political leadership who aren't interested in creating an environment where workers are rewarded in line with their productivity.