If you cast someone in their late fifties or early sixties as Higgins, Laura Benanti would look just fine alongside them. And yes, Kelli O'Hara played the part when she was around forty and was just fine. And she looked forty in the part too, a beautiful and youthful forty, but still forty. Eliza is an extremely difficult part that requires a crystal clear classical voice and lots of personality. I have said this before and I stand by it, that while Pippa is an amazing actress and an amazing singer, her voice is not quite right for Eliza. She has a bit of a pop-y tone to her voice that causes her upper register to have a nasally quality (again not a bad thing when singing scores like The Great Comet, Hamilton, or even A Little Night Music, which she did at Juilliard) that won't sound extremely pleasant on Eliza. If you want to hear what casting someone with a pop-y voice in that role sounds like, check out the London revival cast album and you will hear why it doesn't work. The role of Eliza will take quite a strain on Soo's voice. Also, I have seen her in basically everything that she's done in NY and she's always very poised. I think she'd be great as Eliza post-transformation, but I don't think she'd be very good as the flower girl. Who knows, maybe she will be cast and surprise me, but I would just like to see a stronger, funnier presence with a more classical voice on stage. Laura Benanti may seem too old on paper, but she really is the funniest and most beautiful woman on Broadway. It would be a shame to see MFL pass by without casting Benanti before she is just completely aged out of the part.