It helped that Georgia Brown, besides being such a great singing actress, was dark-haired and street-tough. She was a tough mama, and that kind of character earned the size of the character’s tragedy. I imagine Lupone would’ve been very good.
But that’s not what Dickens described. Shani Wallis was more the look of Dickens’ Nancy. However, Wallis’ work seemed slight in comparison to what Brown had done with it. Wallis didn’t have the power or the hard edge of Georgia Brown and was, unfortunately, more like a young Doris Day who’d wandered into this criminal world.
I remember reading somewhere that the Fagin-actor had something to do with Georgia Brown not getting cast in the film. That they’d been competitive with each other onstage and he sided against her.