In my opinion, ALL movie musicals depend on the success of Dreamgirls, Across The Universe, and Hairspray. By this I only mean that if all these films bomb miserabely, then I think this will have a negative impact on future movie musicals on the big screen.
HOWEVER, DREAMGIRLS is likely to be a hit. Perhaps even a blockbuster hit a la Chicago. Then it won't matter if Across The Universe & Hairspray fail or not.
That being said, I don't think either of these films will impact Sweeney Todd. I think it'll happen...if its Burton/Depp. If Burton/Depp don't do it, it'll float around in the pool of musicals waiting to have a turn on the big screen, joining Bat Boy, Contact, Sunset Blvd, Mamma Mia, etc.
"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