Oh I don't agree with that. I think all the British dames are finding roles: Judi Dench, Maggie Smith, Eileen Atkins, Helen Mirren, Geraldine McEwan, Joanna Lumley, Brenda Blethyn, Stephanie Cole, Pauline Collins, Julia McKenzie, Annette Crosbie, Penelope Wilton, Prunella Scales.
I'll grant you Judi Dench, Maggie Smith and some others, but the U.S. tends to be more youth-oriented and less interested in "old ladies" on stage.
But half of the women you name (Julia McKenzie, Helen Mirren, etc.) are a good decade or so younger.
Rodgerdodger gives us the best choice, I think, in Berthe in PIPPIN. That would be PERFECT for MacLaine.
"The gods who nurse this universe think little of mortals' cares. They sit in crowds on exclusive clouds and laugh at our love affairs. I might have had a real romance if they'd given me a chance. I loved him, but he didn't love me. I wanted him, but he didn't want me. Then the gods had a spree and indulged in another whim. Now he loves me, but I don't love him." - Cole Porter