Let's be honest. Other than the fanatics who live, breathe and crap this stuff, nobody gives a damn about Michael Keaton. Most people don't even remember Batman '89. Nobody cares! The writers were trying to rely on 'memberberries, but that doesn't matter to the general audience who just wants to go and have a good time for a couple of hours. You don't make the kind of bank that these movies need by appealing to things that most of their intended audience weren't even alive for. Most movie-going audiences weren't around in 1989. They sure the hell weren't around in 1978 for the original Superman. There won't be that kind of attachment for most people likely to be in the seats. The vast majority of people going to see The Flash aren't fans of the comic books. Their entire interest is in seeing a movie in the Snyder-verse, which after this, is dead. They shot themselves in the foot and their box office reflects that.
Anyone with a brain could have told you tnat, and, in fact, we did.