Never said that. You seem to think that all it would take is for someone to come in and be willing to do it for less for them to be hired. That simply isn't true.
I know that it's possible, and I know that it's happened often enough to justify employees to seek union representation to keep it from happening to them.
Either workers stick together to keep employment standards up, or it's a dog-eat-dog race to the bottom of the wage scale.
At a mom & pop operation or at even what's considered a small business, there's usually no need for employee unions. The boss knows his employees and his employees know him. It is, in a sense, a family, and they're all working together for their mutual benefit.
But at a large company, a business so large that an employee can pass his boss on the street and neither recognizes the other, that's when the employee better get together with other employees and form a group to protect their interests, because they are not considered family by their boss, they're considered human capital, a resource, a commodity. They're interchangeable and easily replaceable.
That's why there are unions. That's pretty much the only reason there are unions. They didn't fall out of the sky. They were created by big business through its treatment of those who provided the labor that made them big businesses. They are a response to a very real need, employee advocacy in the workplace.