What would a musical be without its dancers? A saying (of unknown authorship) has been cited in print since at least 1928:
 
“A musical comedy with good legs ought to have a long run.”
 
   
17 January 1928, The Daily Northwesterm (Evanston, IL), “Acorns,” pg. 2, col. 2:
THEY DON’T ALL WEAR STOCKINGS.
A musical comedy with good legs ought to have a long run.
Allyne.
   
Google Books
The Reader’s Digest
Volume 60
1952
Pg. 22:
A musical comedy must have good legs for a long run (Jersey City Journal).
 
2 August 1954, Morning Advocate (Baton Rouge, LA), “Little Liz” comic strip, pg. 5-B, col. 4:
In order to have a long run, a musical comedy must have a lot of good legs.
 
17 February 1977, Evansville (IN) Courier, “Scram-lets,” pg. 29, col. 7:
Another thing a musical comedy must have for a long run is GOOD LEGS.
   
Google Books
20,000 Quips & Quotes
By Evan Esar
New York, NY: Barnes & Noble Books
1995, ©1968
Pg. 732:
Another thing a musical comedy must have for a long run is good legs.