Cirque du Soleil has had a lot of this since 2007, but especially after Criss Angel Believe initially bombed with critics and audiences (though it's since had a Retool and continues to run) and ZAIA had trouble selling tickets in Macau. After two of their 2010 productions (Banana Shpeel and Viva Elvis) had bad press in previews — the former wound up closing in New York two months sooner than planned, and the latter closed in 2012 (their first Las Vegas show to do so) — the fandom and even media commentators began speculating the end for a once-wonderful company is nigh. Things didn't improve with the announcement of Michael Jackson-themed shows, not only for being more Jukebox Musical variants but for coming closely on the heels of their subject's death (making it look like a cash-grab). In response to that uproar, the online fan newsletter Fascination had an excellent column by its editor pointing out that Cirque fans have been making Ruined FOREVER claims as far back as 1990 and their fourth show, Nouvelle Experience, as it was a more theatrical show than the tours that had preceded it. With the thinning of most of the disappointments from the herd, plus some well-received newer shows (Zarkana, Amaluna, even Michael Jackson ONE), matters have improved somewhat in The New Tens.
Andrew Lloyd Webber's sequel to The Phantom of the Opera, Love Never Dies, killed a lot of the fans' love for the original show, to the point that a Facebook group called "Love Should Die" was created. He alienated fans even further by blaming the financial and critical failure of it entirely on the people who've been buying tickets to Phantom for twenty years, accusing them of being unable to cope with change.
Pretty much anytime a major musical is made into a movie, there is some set of fans who declares the show RUINED FOREVER.
The (well-substantiated) rumour that Taylor Swift had been cast as Eponine in Les Miserables (2012) provoked an unanimous outcry from the Les MisÚrables fanbase. Then the incredibly sneaky Cameron Mackintosh announced that the role would instead be played by West End and 25th Anniversary Concert veteran Samantha Barks, and this trope was summarily turned upside down and shaken with prejudice as said fanbase - including names like Lea Salonga, Frances Ruffelle, and Rebecca Caine - dropped to their knees with an audible thump and thanked their respective deities.