You probably already know this, but in the play on which "My Fair Lady" is based ("Pygmalion," by George Bernard Shaw), Eliza makes the opposite decision. You can see the ending at the link below as well as the postscript that Shaw added for published editions that tells (in normal prose) what happens in Eliza's life after the play closes. (If you want to skip directly to that part, do a search on the page for "This being the state of human affairs"; it's long, but it's interesting to read.) I think I remember reading that Shaw would have abhorred the idea that Eliza went back (although it seems he did move somewhat in that direction for the 1938 film, according to Wikipedia). Somewhere else I think I read that someone involved with the musical asserted that Shaw got the ending wrong by *not* having her go back.

I suppose I'm just wondering if, in a play that follows mostly the same course but ends very differently, does your argument hold for that version as well?