I do not believe Mariah wants to go into legacy status just yet, and that's why that isn't happening. I am pretty sure Queen wants at least one more big hit before she hangs up the hit making hat.
Mariah took one step into legacy status the minute she signed onto Idol and then delayed that album. The Vegas residency solidified her status as a legacy act and her whole career sort of centres on Christmas now too, again her legacy.
I don't necessarily think that's what she wanted but she effectively forced it on herself when she started rebelling against the labels, not having stable and effective management after she had her kids etc. The control she sought over her career has been her downfall in one way.
But in another way, her whole life changed when she had her kids and I think she's quite content touring and making money that way and getting huge exposure at Christmas rather than constantly being in the studio.