I've been thinking about the performance of this movie and the ''haha you all thought Keaton would be a big draw'' brigade online.
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I think the reality is Keaton is fondly remembered and cherished by a certain generation (30-55 age range), but the under 30's simply don't feel the same way. The Burton movies are smaller, more intimate and very much quaint by today's standards. They can appreciate them, but it's simply not ''their'' Batman. Not enough crazy action sequences, not enough in your face humor, not enough CGI detours, not enough world building.
WB have spent the last 30 years pushing the ''new'' with very little consideration of what came before when it came to Batman. Keaton morphed into Kilmer who morphed very quickly into Clooney, who was then followed by Bale, Affleck and now Pattinson. The franchise has had its ups and down, but the truth is... it's the character that is popular, never truly the actor within the cowl. Sure, the actor can bask in the glory of success, but it's all about a certain vision of the character hitting the zeitgeist at the right time.
Super hero movies are destined to be rebooted, reimagined leaving the older versions without a true sense of relevancy. If you want to show your kids a great STAR WARS movie, you stick on A New Hope and then allow them to discover the rest. Raiders of the Lost Ark isn't going anywhere, neither is Back to the Future. These movies are allowed to breathe and become truly generational allowing, for example, the return of Harrison Ford as Han Solo greater significance amongst a much wider audience.
Michael Keaton and Batman 1989 does not have that luxury.
If kids want to watch a Batman film now, they have ALOT of options to choose from and most likely will gravitate to The Batman and find ''their'' Batman with Robert Pattinson. Until one day, their own kids will blink and wonder ''Pattinson who?''.
It worked with Spider-Man: No Way Home as they played the ''Will they? Won't They?'' angle and the true hook of that movie was... come see the very well liked current version of Spider-Man actually interacting with two other iterations of the character. They brought fans of each version together (and within a much shorter time scale). Ezra/The Flash is certainly no Holland/Spider-Man.
Now... do a Bat-Verse movie featuring Keaton, Bale, Affleck and Pattinson (with a cameo from Clooney) and you have a very different story. Keaton on his own in a movie featuring a character/actor the audiences have not gravitated to is fighting a losing battle.
I guess us older fans simply got caught up in our own hype.