Grand though his flights are, many of them end with his realizing that he’s tired and cold and lonely and that it’s suppertime.... He has animal needs, and he knows it, which makes him, in a word, human.I don't find Boxer's argument terribly persuasive. There's simply no comparing the philosophy and wit of the 1950s and 1960s strips with the banality of those starting in the late 1970s. Boxer, perhaps inadvertently, includes a great example of the impoverished later strip, a Sunday edition in which Charlie Brown and Snoopy appear. The gag has Charlie Brown taking a typewritten sheet of paper from Snoopy and handing it to his teacher. Charlie Brown remarks, "No, ma'am... my dog didn't eat my homework... He wrote it!" We have, in addition to the by-now well-worn conceit of the dog who can write, essentially a play on words. That's it. That's the sum total of the gag. You have to believe Bill Watterson could have made more of this premise than Schulz did.
Schulz was simply out of ideas by the mid-1970s or so. Yet, perhaps because he felt it would be sinful not to work every day, he kept going anyway.
You can find hidden meaning in just about anything, and Boxer may be right that Snoopy's more existentialist than I care to admit. But sometimes you have to admit what's staring you in the face, and what stares any reader of Peanuts in the face is that the first couple of decades' comics are a whole lot funnier and smarter than the rest. Snoopy might not be the reason for that, but he's the outward sign. If Schulz operated as deeply as Boxer thinks, he did a lousy job of conveying it to the rest of us.