Rodgers & Hammerstein revolutionized the form with "Carosel," "Oklahoma," and "South Pacific," interweaving complex and sometimes controversial story lines involving complicated characters. Indeed the 1950s were the golden age of the musical.
Since then, with the exception of Stephen Sondheim, the form has gone nowhere. Sondheim's characters became even more complex, his lyrics more personal, his music more intimate than Rodgers and Hammerstein. Sondheim's not for everyone, because a lot of people want musicals to be just fun, fun, fun. Sondheim is serious. Listen to "A Little Night Music" or "Into the Woods" or "Sunday in the Park with George," jus to name three of my favorites and you'll see!