Related to this "good reading" criterion is this: Classic works are indispensable. Each may give us something we can't get elsewhere. If you haven't read and digested Hamlet, you cannot get that flavor somewhere else, except perhaps incidentally or mixed with other flavors. If Blake's Songs of Innocence and of Experience disappeared, nothing else would make the loss bearable. The same, I suppose, would be true of other classic works. Conversely, I don't suppose that popular thrillers are indispensable. You can get pretty much the same thing from Book X as from Book Y.
I don't know that I'd agree with this part. I think of an author like Hammett or Chandler... both wrote classics of their genres. Their work has had appeal for multiple generations and spawned legions of imitators, providing the noir template for decades of authors and filmmakers. But I wouldn't consider them indispensable, nor would I say the same about Doyle or Agatha Christie. Detective novels with similar flair would still exist... without Doyle and Hammett, we'd still have Christie and Chandler. Without any of the 4, we'd still have Collins/Poe and Westlake/MacDonald. And regardless of any ONE author, we'd still have inspiration for modern detective authors.
This becomes even more troubling the further back we go. Do we KNOW we couldn't get the flavor of Shakespeare elsewhere, or is it simply that he's the one that got written down and republished most at a time when print was at a premium? For that matter, we're not even sure he WROTE all of his plays or if some of the plays credited to him might have actually been written by some of his peers.
I think, particularly with "classics" from before the time of mass market cheap publishing, some books earn classic status through dumb luck. Being the one text a monk managed to grab from a burning village. The playwright whose rich benefactor was eccentric enough to invest in copying his works. The Norse legend that still got told because all the other tribes were slaughtered by the people telling it. And yes, some got preserved BECAUSE they merited it.
Now we're in a new era where preserving EVERYTHING seems possible and, as some have pointed out, classic becomes a tactic to drive sales of repackaged things nobody cared to reprint for 50 years (Library of America series, some of the SF Masterworks, some of the noir authors I mentioned above) but that are now being sought nostalgically.
Then again, sometimes this is because devotees see gold the masses missed... I think of the old joke that only 500 people bought the Velvet Underground's first album, but every single one of them started a band.
Classics aren't indispensable (or original to use the music snob version of the same concept), so much as they're the best example we have of what they do. We would still have great mysteries without Dashiell Hammett and great romantic poetry without Blake, but because multiple generations of the people that love mystery stories, or plays, or poems all tend to agree that Hammett, Shakespeare and Blake did those things as well as just about anybody despite 50-500 years of people trying to outdo them, we call their works classic.