"Of the 50's and 60's, all we know are the "classics" that get revived."
This statement hints at why certain plays and musicals are regarded as classics and hold a place in the musical theatre/theatre canon--they stand the test of time. They keep being produced. They keep being discussed.
"Even the most musical-saavy of us has not seen enough musicals to constitute best and worst in history"
This is why, thank God, we have a collective theatre community that has for the last century cultivated this consensus. Advised us to see certain musicals and plays. Passed down information. This is the only reason we know anything about plays from ancient history.
"You can't judge THE MUSIC MAN against WICKED, for instance, because they're completely different and from completely different eras. Furthermore, MUSIC MAN has had HOW many years to become a classic?"
And here you touch on the belief that it takes 100 years to determine whether or not a play or musical is indeed ART. The farther away we are from something's genesis, the more objective we can be and the more the writing is tested. Who knows what musicals will be remembered in 1000 years? But the ones that are remembered are the ones that will matter. All these years later (even if not yet 100), certain musicals are thriving where others have disappeared. And, it isn't always about quality, as we all know. Many would say Grease is not a finely crafted musical, but it is one of the most produced musicals in the world. It has earned its place in the canon. We will certainly debate (as this thread was started for such) whether or not contemporary musicals and plays will or won't be added to the canon/survive 20, 50, 100 years. Do I think Wicked will be around then? Yes. Do I think it belongs in the canon for the criteria I have mentioned? Yes.
"And also, the fact that none of us are 100 years old and can remember what, in 1956,..."
By that logic, I have no right to claim that Hamlet is a classic because I wasn't alive when Shakespeare wrote it.
Clearly (as I mentioned above), we have hundreds of years of productions, critiques, and the script/productions in front of us to evaluate whether or not Hamlet is a classic. But, at some point (and the point I am trying to make about Oklahoma!, The Music Man, and Kiss Me Kate), the decision on whether or not a play or musical is a classic is a done deal. Hamlet just IS. And, The Music Man just IS.