Now I'm not a super-informed comic historian, but Batman has changed significantly since the start. I think perhaps Bruce Wayne's character being 'a millionaire playboy) was simply the creators originally trying to explain 'realistically' a character who could afford all these toys and the time and energy to fight crime at night.

Sort of, not really. He was originally rich because he’s loosely based off of Zorro, who was loosely based off the Scarlet Pimpernel and The Count of Monte Cristo, who are rich aristocrats. Being an old-money aristocrat of European descent is kind of important to Batman’s gothic byronian hero thing. If they reimagined him as a nouveau-rich tech bro billionaire it would make the character fundamentally different.