To play devil's advocate for the people wanting a sequel, the concept of a shared universe is a relatively new thing and I think it adds a new dimension to sequel-making that complicates things a bit.
It kind of (sometimes unfortunately, in the case of SS or BvS) creates this obligation to develop more content in a way that has never really been implied in film before. It also helps that superheroes are already so pervasive in so many other forms of media and entertainment.
CA: Civil War ends on a note that implies a continuation of not only its main character's story, but threads into multiple films. It warrants a continued story simply by the virtue of its existence. Like, hypothetically, if Thor 3 bombs, Marvel isn't gonna go "well shit, guess we have to shut everything down".
In other words, creating this shared universe also creates a burden to sustain it; a sort of self-fulfilling prophecy. WB/DC is 3 films into its universe and arguably has yet to make a good one that really warrants its own existence, but it's a huge investment so they're not just gonna drop the whole plan because all 3 have been critical failures. And I'm not talking about revenue, I'm talking about sheer cultural impact and viewer/fan satisfaction, which has been mixed at best.
Compare that to something like ... I dunno, John Wick 2, which implies a continuing story, and is clearly part of a trilogy, but doesn't automatically warrant a sequel simply because it was made.