Actually, it didn't have to be a popcorn flick. It just had to show more contrast between the two characters, not just conflict. It was so completely ironic when two of the people who worked on BvS said something in an interview like "There has to be contrast, it can't all be just light or dark." And I just kept thinking of Inigo Montoya's line from "The Princess Bride: "You keep using that word. I do not think it means what you think it means." And then Terrio said at the time, of JL: “I expect Justice League will be tonally not quite as dark as Batman v Superman,”. The problem is that I and a lot of others keyed in on the two most important words of that sentence: "not quite". Everything that was said of these characters could have been said in a way that audiences would have been good with and nothing would have been lost.
That's a big one. Many of us as fans can fill in the blanks with as much of a "Superman we know" as possible and that helps, but general audiences saw what they saw and it wasn't a Superman they saw as Superman. He's not just any other Superhero, and he can't be treated as such. And in the end, the reactions bore that out. And before someone takes that beyond what I'm saying...no, it doesn't mean Superman can only be one rigid thing. But there are boundaries, especially in film.
It's even worse for Superman. It's not like he gets a shiny new take in movies every 2-3 years like Batman seems to (post "Batman & Robin" notwithstanding). Superman had "Superman IV" (1987), then "Superman Returns" (2006), and then "Man of Steel" (2013). And "Superman Returns" wasn't so well-received, either. So that's almost 3 decades without a great Superman movie, mainly because WB doesn't understand it's own character and won't let actual comics writers (who do) take enough of a crack at it.Waiting two films for a fully formed Superman is one thing. Having to wait four is goddamn lunacy, especially when he has to share screen time with guest stars.
You make the money and build trust, THEN start experimenting. The whole "DC is copying Marvel" complaints were annoying, but Wonder Woman showed that that wouldn't have mattered all that much in the end.
Deborah Snyder was asked about the reaction and said “The main thing we learned, I think: people don’t like to see their heroes deconstructed.” This is partly true and partly bull, but my first thought is that they don't seem to know the difference between deconstruction and demolition. It's like the mechanic who finds "extra parts" after he takes apart and "fixes" your engine: it means there's something missing, and it's probably important.