I think the movie and the stage show are completely different projects and artforms, which basically have nothing to do with eachother. When I decide to watch the musical, I want a young Deloris, with an excellent singing voice, as that is what makes or breaks the scenes, the intention of the story, the material, the music, the chosen language and the chosen artform and the notes. Basically the way the scenes, the music, the show was written, intended and created and how it was sculpted and cast accordingly.
There are people visiting the show who love the music and the voices as the show itself has a history too and will be presented with this instead. Some people may neigh loudly when they see a celebrity and forget about everything else, but there are also many people who respect the show and are looking for very different things. I think this casting is a bit disrespectful.
It's like casting Jamie Bell as young Billy in Billy Elliot The Musical in the West End. 2 completely different artforms, generations, scores, music, different languages of storytelling, it is lunacy to me.