I guess this makes me something of a fatalist, but I'm grateful for Batman 89 being made exactly the way it was for the reasons I stated earlier. If it had been an origin movie...A. It wouldn't be the more mysterious take on the character that many still hold dear, and B. We would've likely never gotten TDKT.
Ultimately, I think a director like Chris Nolan was more suited to tell the journey of Bruce Wayne into Batman, just like a director like Tim Burton was more suited to portray him as a dark, mysterious creature of the night. Not that Burton couldn't have done it, but just try to picture a Burton movie (especially from early in his career) with ninjas and Bruce Wayne having a sword fight on a frozen pond out in the Himalayas. It just doesn't quite compute. There's a sense of straight-forward earnestness and adventure that comes with that type of movie and Burton is more about dark humor, the bizarre, and creating his own idiosyncratic world.
But who knows, if Burton had come back for a third movie, maybe Keaton would've gotten his way and they'd have done a prequel.