Steel Pier Fan:
Some of our best musicals have been based off of movies: A Little Night Music, Sweet Charity, Carnival, The Full Monty, 42nd Street, Grey Gardens, and Promises Promises to name a few.
Some of our worst musicals have been new and original: In My Life, Merlin, Starmites, Jennie, Marilyn, The Pirate Queen, Mail, Hot Feet, and Baby It's You to just name a few others.
Where it comes from doesn't matter. If it's given to talented people and the source material is strong, there are endless possibilities for a great musical. And the truth is, is that the "original" Broadway musical is a rare species. Most musicals for the past 80 years have been based on SOMETHING (Play, book, album, life story). But since you seem keen on knowing that the "original" Broadway musical is alive and well, think about this: in the last 7 years, we've seen The Drowsy Chaperone, The Book of Mormon, Caroline or Change, Avenue Q, The 25th Annual Putnam County Spelling Bee, Fela, The Scottsboro Boys, Curtains, Next to Normal, In the Heights, and Passing Strange. That's only including the decent to great original musicals (there are countless other fair to terrible original musicals that have premiered in the last 7 years).
The Broadway musical is fine.
"Sing the words, Patti!!!!" Stephen Sondheim to Patti LuPone.