So apparently in 1989 when Batman was coming out, there was a group of fans who disapproved of the casting, and you can find interviews and stuff today used to show how wrong they were.. but here's the thing, in my opinion, they were and still are right.
Is he the worst ever? No. Not by a long shot. But he's like 5'7" and not particularly muscular, curly hair, bit wimpy, he just doesn't look like Bruce Wayne to me. Maybe it's better to look at this as it is, which is a new take on the character. But to me Bruce Wayne is canonically 6'2" and is supposed to be kind of a hot man in a ladyboner way. Remember he's a playboy billionaire.
In my mind, the only reason Michael Keaton is so beloved as Batman as he is, is because he was the first one to really be on the big screen for it. Nostalgia.