I'm not worried. I'm actually very intrigued to see what new direction Depp will take this character in. My feeling is that it will be ok to have him not singing as legit as previous Sweeneys such as Cariou and Cerveris, because I doubt the other cast members like Rickman and Carter will be singing Legit. In other words, if Sweeney is the only character singing operatically, it will sound out of place and humorous.
I think this movie is going to be quite good, and not a disappointment to open-minded Sweeney Fans.
"I'm an American, Damnit!!! And if it's three things I don't believe in, it's quitting and math."