I, too, have been surprised by the lukewarm attendance at the time of the approach of the holidays when attendance is usually strong. South Pacific played just short of two years, and most, including critics, who have seen both productions have trouble choosing the one that they most enjoyed.
Some have said that TK&I is a little stale versus SP since it has been revived more often. There was the 1996 production starring Donna Murphy and Lou Diamond Phillips that played 980 performances, long enough to see Marie Osmond take over as Anna from Faith Prince. In 2003 there was a popular production at Paper Mill.
Maybe we are just seeing the dumbing down of Broadway. The first great transformation of the Broadway musical was led by Show Boat and Oklahoma. The "book" musical became essentially a play enhanced by music (see Something Rotten).
Now, beginning with the arrival of the jukebox musical, we may be moving again into a darker period. Not that Jersey Boys and Beautiful are not wonderfully entertaining, but these shows do not have original scores and are driven by the audience response to all the hit songs that they already know.
If these shows and others with no serious book to speak of are now mainstream Broadway, then perhaps Broadway is gradually ending its era of the book musical, which lasted roughly from Hammerstein and his collaborators to Sondheim and his book writers Lapine and others. The musical has become as light and airy as the musicals of the twenties and thirties, where audiences (or should I say the mass audiences) do not have to struggle with a book or an unfamiliar score that they might not like.
Broadway is too expensive for many to gamble on a show that they might not like. The majority of the current pool of theatergoers has been raised on a trashy pop culture, whereas at least much of its predecessors listened to trained musicians like Benny Goodman and Louis Armstrong and most of its pop hits were written by skilled composers who had emigrated to the United States to escape oppressive governments.
I think that Kelli will probably exit The King and I in January, or at least by February. No point in hanging around performing to half full winter audiences when she has a hundred other offers to consider.