One "libertarian assumption" is that it's worth doing a little math to figure these sorts of things out. Usually if you do that math you will discover that neither profits nor high top salaries are anywhere near large enough that companies could merely absorb the higher cost out of either or both. (Incidentally, a raise to $15/hour is quite a lot more than "the smallest hike in pay".)
But hey, just for the sake of argument let's pretend you COULD take it out of profits. Here's where you run into a related assumption which is one of the basic rules of economics: change happens on the margin. Suppose that the entirety of the McDonalds chain as a whole is profitable. For simplicity we'll ignore the fact that the individual stores are mostly franchises and pretend they're all company-owned. The chain is not uniformly profitable! Some outlets are very profitable and others are just barely scraping by. When you increase the cost of labor the outlets that were very profitable are still worth running even with the new cost of labor, but the outlets that were only slightly profitable before are now losing money.
Those are the outlets that close.
Restaurants that will lose money every year with the higher wage aren't worth keeping open. shifts that lose money at the higher wage also aren't worth keeping open, so hours of work tend to get cut back too. You don't lose jobs at every store or uniformly, but you do lose marginal jobs that did exist or that would exist were it not for the new wage.
Overall, the chain will survive, but it has to shrink back in order to do so. In order to argue that it doesn't, you'd need to believe not just that the company overall is making (a lot of) profit, but that EVERY individual outlet is making the same share of that profit, so they're all worth keeping around post-change.
But hey, just for the sake of argument let's pretend you COULD take it out of profits. Here's where you run into a related assumption which is one of the basic rules of economics: change happens on the margin. Suppose that the entirety of the McDonalds chain as a whole is profitable. For simplicity we'll ignore the fact that the individual stores are mostly franchises and pretend they're all company-owned. The chain is not uniformly profitable! Some outlets are very profitable and others are just barely scraping by. When you increase the cost of labor the outlets that were very profitable are still worth running even with the new cost of labor, but the outlets that were only slightly profitable before are now losing money.
Those are the outlets that close.
Restaurants that will lose money every year with the higher wage aren't worth keeping open. shifts that lose money at the higher wage also aren't worth keeping open, so hours of work tend to get cut back too. You don't lose jobs at every store or uniformly, but you do lose marginal jobs that did exist or that would exist were it not for the new wage.
Overall, the chain will survive, but it has to shrink back in order to do so. In order to argue that it doesn't, you'd need to believe not just that the company overall is making (a lot of) profit, but that EVERY individual outlet is making the same share of that profit, so they're all worth keeping around post-change.