Count me in the Worsham camp. While she's only 6 years younger than Kelli the fact that she's only within the past 3-5 years came into prominence on the main stem gives her a "younger" vibe.
Kelli has proven that she's shrewd when deciding what roles to tackle and I think (from an outsiders perspective) the fact that she did the concert with KG could wind up working against her deciding to accept the role. She's very much an actor's actor and will always eschew whatever seems easy for what's going to stretch her abilities.
To my unprofessional opinion this role feels both unnatural in that she doesn't read as Cockney street tough to me and habitual in its "Kelli O'Hara in a revival of a Golden Age Musical" sense. At this very critical point in her career, post Tony win after so many nominations, the pessimist in me is fearful that there are a lot of people out there waiting to jump on her next project with claws out. I'm sure she and her team will choose whatever it is with even more care than they usually do. As kind as the golden age catalogue has been to Ms. O'Hara I'd love to see her in something new, a revival of something more current, or probably most intriguingly for my money a straight play. Has Roundabout begun casting The Cherry Orchard yet? It's been ages since I read it, I'm foggy as to the ages of the characters, but Kelli feels like she'd be right at home with Chekhov.
All that being said I'll echo others sentiments that when Dame Julie calls you to the table you don't leave...