17 years ago - Billy Batson is born.
8 years ago - 9-year-old Billy is taken in by his foster parents, where he meets his fellow foster kids Mary Bromfield & Freddy Freeman, and their foster Uncle Dudley. Billy takes an interest in radio & starts his own podcast.
6 years ago - 11-year-old Billy is guided to the Wizard Shazam by his father's ghost, becomes the Wizard's champion Captain Marvel, regularly battling Black Adam.
3 years ago - 14-year-old Billy's age is discovered during the White Martian invasion & he is removed from the Justice League.
The legacy of Captain Marvel is mired in a lot of convoluted legal trappings. The fact that DC sued the character out of existence and then later bought the company that made him, losing the rights to the name of the character in the interim is the sort of nonsense you couldn't make up. Despite all that, the character works so well he's still here. There have been a few changes to the costume and concept in the new 52, but they're largely pointless.
One element of the character that we're altering is the extended Marvel Family. Created by Otto Binder, the writer who ushered Captain Marvel to the height of his success in the 40's, they were an extended cast of characters that shared Billy's powers. After DC shut down Fawcett comics, Binder worked for Timely, which would one day become Marvel, and for Quality Comics, which would also one day be acquired by DC. Eventually, he worked for DC, where he crafted a extended cast of superpowered characters for Superman similar to the ones he had created for Captain Marvel; including Supergirl, Krypto, Beppo the Super-Monkey...
You see where this is going. The idea of legacy characters, the central concept the entire DC universe is based on, comes largely from Otto's work on Captain Marvel. You could also argue that it owes an equal debt to Bill Finger's invention of the teenage sidekick when he created Robin, but the point is, the Marvel Family is important.
Still... comics have spent a long time removing some of the more comical elements of the pre-crisis stories. We make an effort, when re-imagining these characters, to ask what version of the character best suits the world we're trying to create. While the Marvel family has a place historically, Captain Marvel himself isn't exactly a better character for having a group of duplicate characters around him. Mary Bloomfield and Freddy Freeman are at their best when they are a humanizing supporting cast for Billy. The work we'd have to do to make those characters make sense doesn't serve the larger purpose of supporting the central character.
So Captain Marvel is a singular character, and one of the best characters in the DC stable. As he should be.