Let us posit a few assumptions to what I am about to say.
The first assumption is the following: We can agree (roughly) on the existing cannon of Western Drama. That canon would be, roughly, what gets taught in 100 and 200 level theatre courses, which has little variance, frankly, from school to school. I will, in other words say, that we have some general idea of what plays fit the quality of masterpiece.
Now, let us go to a second assumption: We do not want the current roster of masterpieces to be frozen, we instead want to continue adding to it.
The question becomes... how do we get there? How do we create the plays that future generations will study, or rebel against, or carry the torch for?
I would suggest the answer is produce the ok-to-good plays by today's playwrights.
Over the past couple of years, I've met a lot of young-to-mid career writers. Basically, the young writers who don't self-produce get zero productions and the mid career writers I've met get surprisingly few productions. THey instead get a lot of readings, the occasional commission, a lot of things that fall apart at the last minute etc. There's lots of talk about this in Outrageous Fortune, so I won't bother beating the point to death, but I'll just say there's a lot of effort that goes into doing lots of things that are supposed to start down the path to full production but never get there (and just to be generous, I'll count fully produced workshops like Clubbed Thumb's Summerworks as full productions, even though they only run a weekend).
I was strolling through wikipedia today, and it's worth noting how many plays many beloved playwrights had written and produced before their masterpieces. Because we ignore the first half of the saying "took my whole life to be an overnight success," this gets obfuscated. Yes, there are some Young Geniuses... but there are a lot of Old Masters. My favorite example of this being Tony Kushner, who people seem to think wrote Bright Room and then Angels but had, in fact, written (and produced) thirteen plays prior to writing Millennium Approaches. Sam Shepard had over a decade of film and theatre writing experience prior to Buried Child. Tennessee Williams wrote eight plays before Streetcar. Eugene O'Neill had been writing for twenty years before he wrote most of what is considered his major works (and don't get me started on Moliere or Shakespeare).
Now obviously, this is hardly scientific... I'm just saying that perhaps in asking new plays to cross a Hurdle of Greatness before they can get produced, we create an environment in which writing an actual Great Play becomes nigh-impossible except for the Child Geniuses of the playwriting community.