I think the reason he was originally kept for AG is the show as a whole was only 5 years old at that point. You have to remember, the show was still at the beginning of its history back then. It made sense to keep one focal character throughout it all.
However once we past an entire decade, it started to be obvious Ash was running his course.
BW is the closest to a reboot we've gotten, but since Ash and TR are still here its not quite there yet. If Gen 6 pull a TRUE reboot, it'd be much better off.