This is an interesting topic. I think some form of more minor revision may be warranted or maybe a separate defender rating. But the real question is whether having a higher CP actually makes certain mons good defenders. And it doesn't. There are a small handful of defenders that would be placed if there were CP changes that would be any better than the ones we have. So I would hit a gym in the morning and say "neat" right before I skull f%#€*^| an Onix. That would get old too.
The truth is besides this defenders need to somehow get better but not impassable to newer players which is a crazy hard thing to figure out.