Weingarten's harangue is typical of prescriptivists, who in my experience tend to be scarcely aware they're even advocating a philosophy, let alone one widely rejected by lexicographers and linguists. They present their criticisms of the way people speak and write as nothing more than commonsense conclusions that they remember better than others because they stayed awake during third-grade English. Rarely do prescriptivists question any of the traditional rules they were taught in school, many of which do not hold up to scrutiny. They are discussed at length by the linguist John McWhorter in his 2001 book Word on the Street, which presents a wealth of evidence that many of the so-called "rules," from avoidance of split infinitives to the prohibition on using they with a singular antecedent (as in "everyone returned to their seat"), are rooted in the basically arbitrary decisions of a group of 18th- and 19th-century writers who often had a poor understanding of how English worked. But because these rules have been taught to generations of schoolchildren as ironclad truths, educated people have come to think of them as being on par with the laws of thermodynamics.
That's why no evidence from history or literature or any other field can possibly sway the fervent prescriptivist. Consider how Weingarten addresses the fact that many classic writers such as Jane Austen adopted the nonliteral literally on occasion: "That no more makes it right or acceptable than it makes it right for you to annihilate 100,000 people with a bomb just because Harry Truman once did it."
With this statement, Weingarten joins the honorable company of the critic John Simon, who wrote in 1980 that "The English language is being treated nowadays exactly as slave traders once handled the merchandise in their slave ships, or as the inmates of concentration camps were dealt with by their Nazi jailers." Most language scolds I've encountered aren't quite this colorful in their choice of analogies. A professor of mine made the point more simply when confronted by evidence that a usage he disapproved of appeared in the works of great writers: "It's still wrong."
The real problem with this argument is that it assumes a word's proper definition is some immutable law of nature, like gravity, that can never be shaped by the people who use the language, not even by the people who use it best. This view is positively blinkered. There's no reason why the English of Shakespeare is different from that of Chaucer, or from that of Weingarten, other than that human beings of every generation have spoken and written differently than their predecessors. And if there is one thing linguistic history absolutely makes clear, it is that today's error is tomorrow's rule. For example, nice once meant "foolish." It evolved to its present state because people kept using a "wrong" definition, but it's hard to see how English suffered as a result.
Of course, literally isn't just any evolving word. Its traditional definition is a useful concept to have a word for, and it would be a shame to see it go obsolete, which may happen if more and more people say things like "He literally puts his money where his mouth is." In that sense I'm with Weingarten that the looser definition should be avoided (though not excluded from dictionaries). What's striking is that he never makes this argument. His point is simply that it's the law, and we must obey. His indifference to judging word usages based on their utility is revealed in his offhand comment, "although I may cringe at 'blogosphere' and 'webinar' and, sigh, 'whatevs,' I do not protest their appearance in dictionaries." Now, why would anyone cringe at a coinage like blogosphere? (Least of all a blogger?!) Only someone who believes that language should remain literally frozen in time, and that all change is bad, would find anything wrong with that type of innovation.
Weingarten doesn't even accurately explain the loose definition of literally. He claims it is being used to denote its opposite, the word figuratively. It is not. As the OED notes, it is being used as an intensifier. It's basically a synonym for really or actually, except that those words have been blunted from overuse, so when you want to express that you really, truly mean something, literally sometimes gets the point across with more force.
Hence, "the coach literally hates my guts" is meant to convey that you aren't exaggerating the coach's hatred. In a way this is a form of traditional literally; it's just being applied selectively, to the level of the coach's hatred rather than to the metaphor used to describe it. What this example shows is that a statement can have multiple layers of presumptive nonliteralism, and literally may be intended to unpack one layer while leaving the next alone.
My point here is not that I approve of the loose definition of literally, but that it isn't necessarily based on ignorance of the traditional definition. Rather, it's a reflection of the fact that our language is littered with dead metaphors that are all but invisible to us. (The mixed metaphor I just used is further evidence of that fact.) This helps explain why the traditional definition hasn't disappeared from the language, despite centuries of being disregarded. Annoying as it is, the loose sense has come to coexist alongside the traditional one instead of replacing it outright. Weingarten misses this point when he quotes Ambrose Bierce's supposedly accurate prediction that "within a few years the word 'literally' will mean 'figuratively.'" In fact most people today use literally in exactly the way it was originally intended. We just pay closer attention to the loose sense because of the way it literally sticks in our craw--suggesting the danger it poses to our ability to communicate may be overstated.
That's actually true of most gripes about language usage. Some are completely groundless (the most famous being the split-infinitive "rule"), while others, such as this one, at best point to bothersome trends that detract from our language's vitality. In neither case is any large-scale damage on the horizon. As McWhorter explains in his book:
What we must realize...is that during these changes, because renewal always complements erosion, all languages are eternally self-sustaining, just as while our present mountains are slowly eroding, new ones are gradually being thrown up by the movement of geological plates. Thus at any given time, a language is coherent and complex, suitable for the expression of all human needs, thoughts, and emotions. Just as linguists have encountered no languages that do not change, they have also not encountered any languages whose changes compromised their basic coherency and complexity. We have encountered no society hampered by a dialect that was slowly simply wearing out like an old car. Anthropologists report no society in which communication is impossible in the dark because the local dialect has become so mush-mouthed and senseless that it can only be spoken with help from hand gestures. In other words, there is no such thing as a language 'going to the dogs'--never in the history of the world has there existed a language that has reached, or even gotten anywhere near, said dogs.