Most of the DCAU's greatness was Bruce Timm telling the same executives that have always botched up everything to fuck off or alter their shit ideas.
Execs: Alright we've already had you up the cartoony-ess with the New Adventures of Batman but it's time we appealed ONLY to them kiddies!
Timm: but it does now, that's why we went with Tim Darke
Execs: No you don't get it Batman is the only one worth a damn but he needs to appeal to kids, we know! BATMAN IN HIGH SCHOOL
Timm: Ok let's set it in the future have older Bruce Wayne mentor a "new" Batman and tie it in to the other series
Execs: BRILLIANT, glad we thought of it
and thus Batman Beyond became a thing.