I keep reading takes about why The Flash flopped and I'm waiting for somebody to note that the character has been available for free in a nine-season, 184-episode series that ended three weeks ago. Movies and TV are not separate planets, as much as the movies wish they were.
Also, for hardcore "I need to see every chapter of the saga" fans, The Flash is what happens when you announce more than a year in advance that the current extended-universe thing (which is now a mixed selling point at best) is a big miss and nothing counts until the redo in '25.
I think that both Marvel and DC, though their levels of success are obv. different, are coming face to face with exhaustion with the "It's all one big story" aspect of the genre. For most moviegoers, it's become about characters (Spider-Man), not about chapters (Ant-Man).
DC's attempt to revitalize saga storytelling across TV & movies could work. But given Shazam and The Flash, it feels like the co. has reverted to an old norm that lasted decades: Batman is its own special thing, and everything else has to settle for various levels of not-Batman.
One more thing while I'm musing about this: Spider-Man: No Way Home was not a smash because everyone suddenly loves multiverse stories. It was a smash because it was fun, emotional, and incorporated 20 years of multi-generational fan love of the movies. That trick works once.