I think it's pretty safe to say that theatre is the only place where sequels never work.
Think about it: Can you name one sequel -- or, for that matter, prequel or spin-off -- that has not only been successful, but has actually been good? (Let's not confuse those two qualities.)
Didn't think so. (And before you mention Falsettoland, may I remind you that, originally, March of the Falsettos and Falsettoland were one show.)
What is it about sequels that doesn't coalesce when it comes to theatre? Specifically, sequels to musicals, because there have been plays that have had fairly successful additional chapters (i.e. Neil Simon's Brighton Beach Memoirs/Biloxi Blues duology, A Doll's House Part 2, and more recently, Gary: The Sequel to Titus Andronicus).
But second helpings of musicals? Not so much.
Here's why: the stories are terrible, the songs are forgettable, and too much time passes between parts one and two.
Consider the three sequels Bring Back Birdie, Annie 2, and Love Never Dies.
Bye Bye Birdie was one of the biggest Broadway blockbusters of the 1960s, but it should have followed the advice in its title and bid a fond farewell at the end of Act 2. However, as in any industry, when something is a cash cow, you sometimes go back for more of that golden goodness.
It takes a long time to write and produce a full staged musical, but in Birdie's case, it was too long. Almost 30 years passed before Bring Back Birdie made it to the stage, and Star Wars this was not; by that point, no one had any unanswered questions from the original or was clamoring for more stories about the obnoxious pop star. Those who grew up on the original, perhaps performing it in their stuffy middle school auditoriums -- and who were now adults -- realized that maybe the original wasn't actually that good after all, and even for the 1980s, felt a few of the themes to be a tad problematic. I mean, Birdie turns out to be sort of a dick, so why would audiences care about him now? It's not like the creative team was going for a Breaking Bad/Sopranos-style character redemption. Conversely, anyone too young to remember Bye Bye Birdie probably felt that a sequel wasn't worth laying out the big bucks for tickets since they didn't know the origin story.
Oh, and as much as Charles Strouse, the songwriter of both Birdies, is a legend in musical theatre, this was perhaps his most unforgettable score. Trust me, I've listened to Mayor.
Bringing back Birdie didn't deter him, though. Only a few years later Strouse returned with Annie 2: Miss Hannigan's Revenge, which sounds like an action movie parody (and, to be clear, it wasn't). Centered on the incarcerated titular character, rather than the adorable redhead orphan, and her scheme to scam Warbucks, they bizarrely omitted her partner/brother-in-crime, The Rooster. It's therefore a surprise this got green-lit; after all, Hannigan is a fun character in small doses, but the central role in a show? Despite its flat-out failure to meet even a modicum of the original's thrall, its creators demonstrated the same pluck as Annie herself and soon reshaped it into Annie Warbucks. This time around, they completely scrapped Hannigan altogether (there goes the baby with the bathwater) and subbed in a plot line that pushed suspension of disbelief into unheard of territories; in order for Warbucks to keep Annie as his adopted daughter, he had to get married. Poor Grace Farrell, pressured into marriage to a bit of a lout just to save Annie.
Even though this iteration of the show was slightly more successful than its predecessor, it was a huge loss, especially for Strouse, who this time wrote some pretty good songs for it ("Above the Law," "The Other Woman," and an overlooked gem, "Love") that deserved a better show.
Bring Back Birdie and Annie 2 might have had a few problems, but Love Never Dies -- the sequel to The Phantom of the Opera -- fell victim to all of the missteps for Broadway musicals. And then some!
For many theatre-lovers, you can't do much better than Phantom; the story and the music come together in just the right way to form a near-perfect musical. It's head-scratching, then, that 24 years after its first Broadway bow, Andrew Lloyd Webber and company went back to the well. They would soon find out it had run dry, basing this far subpar sequel on the novel The Phantom of Manhattan. Much like Bye Bye Birdie, the end of Phantom was satisfying. Mysterious? Yes. Leaves the audience wanting more? Yes. But that doesn't mean they should get more. It's not like Webber was hurting for income, even though his last few forays (The Woman in White and Whistle Down the Wind) ) had not been box office winners. When Love Never Dies was DOA, he must have been in a bit of a panic.
The only reason I can explain why Love Never Dies was given life was that the creative team saw Phantom's unending existence on Broadway, thus assuming audiences couldn't get enough of the masked man. Where they erred was that crowds just wanted more of the original they'd either seen before or grown up hearing about, not a fan-fiction imitation.
Since the demise of Love Never Dies, the nail has stayed in the coffin of Broadway musical sequels. That may not continue to be the case, though, as The Greatest Showman is prepping not only a Broadway version but a film sequel. (I sense this won't suffer the same fate as the aforementioned shows since it was originally a movie.)
Broadway producers (with the exception of Bialystock and Bloom) can take the following lesson from these failed experiments: When a show is beloved by millions, leave it alone. We'd much rather revisit our favorite characters in their original time and place, even if it's the same old song-and-dance.