Why Custom of the Country is not one of Wharton's great novels

So at the end of Edith Wharton's The Custom of the Country, Undine is just as evil and narcissitic as she was at the outset - she doesn't change at all over the course of the novel, over the course of 3 marriages and one re-marriage, over the death of her husband, over the demise of her father's fortune, and so on - events happen all around her but nothing changes her, she's adamantine - which altogether explains why this novel is entertaining enough to read and full of brilliant observations and turns of phrase but is not and never will be grouped among Wharton's best. Granted, it would be cheap and easy out to have Undine discover true love at last and become a caring spouse and mother and daughter and citizen. And it would be an easy out as well, and maybe too depressing, to have her fall on the rails or load up on rat poison, like other famous sorrowful heroines. But we want to see the main characters grow or change over the course of a novel - every novel is a journey, in a sense, from innocence to experience - and some characters in this novel do change and suffer and learn from their mistakes, or not - but not the main character. So the ending struck me as very flat and empty - could almost open up into a second volume, Undine's attempt to become part of the embassy set - but it ends, oddly, with her finally with all of the wealth she could even imagine. To things do strike me there about her odd relation to Elmer Moffat: is it believable that she would have married him the first time, back in Apex, when he was such an obvious lout, so ill-kempt and crude - he does not seem in the least like the kind of guy she would latch onto to get out of Apex. And then, over the course of his life, he becomes a billionaire railroad tycoon - and art collector - but still crudely vulgar, a parody of an American mogul - and I can certainly see that Undine would be drawn to him for his money, but wouldn't she also be repulsed by who he is and how he behaves? Material there for another novel for sure, but Custom ends with that element unexamined. Definitely a lot of material here - might make a good mini-series - but just compare this flat ending with the beautifully subtle and ambiguous ending of Age of Innocence and you can immediately see the difference between a high-class potboiler and a work of literature