We didn't have to get weaker in old expansions - we got stronger, it's just that the forces we were fighting also got stronger. That all got broken when they started scaling content to your level.
Yes, secondaries got scaled down, but a level 80 in Sunwell gear was stronger than a level 70 in Sunwell gear in every zone. Now, a level 60 in level 50 gear is straight up weaker than a level 50 in level 50 gear as they go into a level scaled zone.