You're assuming a consistency of plotting from a studio that just last year hired a freakin'
trailer company to re-edit Suicide Squad, and which had no idea while making
Man of Steel that Gotham and Metropolis practically adjoin each other.

I find it
much more plausible that the movie's happy, pretty-sunset coda was a reshoot to send the audience out on an upbeat note, rather than the downbeat and narratively consistent one they'd originally intended. Or maybe it wasn't a reshoot at all, and Jenkins was ignoring that clear implication of
BvS from the start. Either way, I'm about zero percent convinced that Snyder
didn't mean for Diana to have turned her back on humanity in general when he made
BvS.