The truck drivers are really going to mess things up. I personally know 5 different truck drivers and they all aren't vaxxed and they all will refuse to get it. Shit is about to get real fucked up.
You're right. I really think tons of workers are going to call their bluff on the mandate. Hiring is tight already for jobs requiring "real" work that keeps the country going, i.e. the "essential" worker "heroes" we were hearing about all last year.
They're pissed off because they see how expendable they're being treated. Horrible move to aggravate an already inflamed situation. Even if 10% of truckers, bus drivers, airline employees, doctors/nurses, etc stop rolling, the disruption will be massive and devastating.
McDonalds will likely be the least affected of all major chains by this. They have continually been acquiring companies to aid in automation and have had fully automated locations for a few years now. Retail stores, large trucking companies, healthcare and international shipping infrastructure (dock workers/crane operators) could bring this country to a grinding halt though.
On the topic of food though, I assume say TacoBell wouldn't require every TacoBell employee to be vaxed because almost all of them are individually owned franchises. The food people will be exempt, but the factory workers and shipment folk won't be, which is what would likely cripple them.