Broadway is looking more like Universal Studios than the epicenter of great theater these days. Mainstream brands are squeezing out scintillating art that once dominated the boards. Exeunt: “Sweeney Todd,” “Gypsy” and “Rent.” Enter: “Margaritaville,” “SpongeBob SquarePants” and “Frozen.” One longtime theater critic told me the last 10 months of shows made up “the worst...