An essay I wrote in 1999 defined five periods in the history of musical theatre:
1) the operetta era, ending in the 20's
2) the musical comedy era, spanning the career of Rodgers & Hart
3) the Golden Age, which one usually ends around Hair
4) Director's Theatre, when the director was often more important than the writers. Quick, it's whose A Chorus Line?
5) Eurotrash, which seems to have begun with Les Miserables
In the twenties and thirties, musicals were fun romps, and the books merely held together a collection of songs the writers expected to become hits. It wasn't important to them to accurately depict time and place, they were just looking for a hit, so you get September Song in Knickerbocker Holiday, a show set in the Dutch period of New York history.
The Boys From Syracuse is a prime example of a musical comedy, words and music meant to make one laugh, and, in the musical theatre of the 1930s, the genre hadn't evolved, yet, to discover the importance of musically dramatizing time and place.
The composers that are writing today should be aware of the innovations of the Golden Age. My trouble with much that's been written in the past 30 years or so is that they ignore the virtues of coloring time and place with music, eschewing self-pity, including subtext and wit, and rhyming correctly.
Luckily, there are still writers around who value those virtues: Maltby & Shire, Yeston, Sondheim, Kander & Ebb, Larry Grossman, Carnelia & Hamlisch, Strouse & Adams, and Jones & Schmidt, to name a few