Route B23 has been around a long, long time. In fact, it was the first trolley coach route in Brooklyn. Even then (1920s!), though, it wasn't exactly a "heavy hitter." Over the years its hours of operation have been cut back, though there's still room to cut in that area. Discontinuing the route altogether is always an option, but it would be a last resort.
The B67 taking over the western portion of the B23 is an interesting idea. It would probably be every other trip during the times that the B67 operates at a 15-minute headway or better (for no worse than a 30-minute headway on the 16th Avenue portion), and every trip at other times. I'd have to analyze it to see whether it would save any money.
David