SomethingPeculiar said: "At this point, there are three women: McDonald, Peters, and LuPone. On a level slightly below, there are lots of others (Chenoweth, Foster, Benanti, O'Hara, Ebersole, Mazzie)"
I have to say that I agree with you re the three leads. Sort of.
Of course, when that show was performed, Mary Martin and Ethel Merman were much more famous than any of the ladies listed is today. IMO Peters and Lupone are well-known beyond Broadway and McDonald is getting there -- probably even moreso Peters because of some movies she made decades ago -- but their fame just doesn't come close to Merman and Martin at their peaks, who were very well known across the country by people who never attended a live show, Broadway or touring.
Lots of reasons: Cast Albums were once huge sellers, certainly true around the time when they sang together...those people who never saw a show in their life had cast albums; there were so many variety shows on Broadway, including Ed Sullivan, and they were the perfect guests...I can't imagine how many times I saw each of them on Sullivan alone...that outlet really doesn't exist today; if you watched TV then, you had a choice of three prime channels (if you were lucky) and a couple of local ones (if lucky). The probability was that more people were going to see them guesting on those variety shows; they grew up when Radio was star; people like Dorothy Kilgalen and Walter Winchell were huge across the country in those days, and regularly covered Broadway.
I have to day that this is not about talent. Both were talented, but a lot of the ladies mentioned are just as talented, maybe moreso IMO. I only saw each live twice -- Merman in Annie Get You Gun and Hello Dolly / Martin in I Do! I Do! and Hello Dolly in London (something that I had to be reminded of by my sister recently, since I had actually forgotten...obviously didn't make a strong impression on a 15 - m16 year old who had already seen Carol Channing). Of course, there was also Peter Pan.
While these may not have been their best performances, I have to say that I have seen many musical comedy actresses in multiple roles who I thought gave more impressive performances in multiple shows than they did in those shows: Lansbury, Verdon, McDonald, Barbara Harris (in only two shows!!), Lupone, Peters, Donna Murphy, Kelly O'Hara, Sutton Foster.
Bottom line: times and tastes have changed too much in the past 60 years. There are no counterparts to Merman and Martin, fame wise, and they would never be given the airtime on a prime TV station, unless maybe on Labor Day Sunday.