That's ridiculous. Adding an understudy role to an ensemble contract is not that expensive. They wouldn't save much, at least not enough to keep the show running. MARY POPPINS has great weeks and then they have not-as-good weeks. They're fine. Pending the box office that moves to MATILDA and ANNIE, MARY POPPINS could probably make it another 1-2 years and be just fine. Knowing Disney, however, they already know when they plan to close it, and when they plan to open a new show at the New Amsterdam.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle