The idea is to get all the McDonalds workers unionized. Sure, they can close them all, but they won't.
What you are missing is the antecedent to "they". In the case of McDonald's, the antecedent is "the local franchise owner". Local franchise owners are not multi-national companies with deep pockets. They run half a dozen locations or so. They have to meet payroll and pay the costs. When payroll exceeds income and the store loses money, they can't just up all the prices, because prices are set by the parent company. That means they go out of business.
Can "they" close all the stores? Absolutely. Our local Wendy's franchisee closed all the stores he ran in this area back when Wendy's parent company mandated all stores close down for a few months for facility upgrades. Our stores closed down "for a few months" to get "the new look", and for us the "new look" was an entirely different company's.
Unionizing a subcontracting shop is pretty counter-productive. It's shooting oneself in the foot. Yeah, great, you can strike and force the contractor to pay more, except he can't get more for contracts he's already engaged in, and he won't get new contracts if he can't meet or better the bids from other contractors. "We're paid premium wages at this shop" ... when we have work to do.