It never needed to be R.

The best Batman movie made wasn't R. It wasn't even PG-13. It was the PG rated MASK OF THE PHANTASM based on the PG and G cartoon, which is one of the definitive portrayals of the Dark Knight, his villains, and his supporting cast.
None of the direct-to-DVD R (and bloody PG-13 but should be R) DC animated films have come close. Because you don't make something more mature and adult just by making it less appropriate for kids...