> Yes, the monopoly argument depends on believing that somehow, even though there were these absurdly high profit margins from simply carrying a different good, no grocer other than WF or WO will ever recognize that for eternity,

No, it doesn't, because antitrust monopolies aren't defined exclusively over an infinite future time horizon; while simple economic models often only consider that, all useful economic models and regulations consider shorter-term effects, as well, or exclusively (“In the long run, we’re all dead”, after all.)