A simple thing they could do is also simply change the damage formula for that moves in general do less damage. They've changed it at least once before I think (between Gen III and Gen IV?). Another thing that happened between Gen III and Gen IV was the physical/special split, which in my opinion was the most damaging thing to defensive play. Back when I started playing, it was simply a case of packing Skarmory, Weezing or Forretress to deal with phsyical attackers (although many started carrying Fire Blast towards the end of the era), and Blissey to deal with special attackers (again, some started using the likes of Focus Punch after a while). However, today you simply can't do that. Skarmory, Weezing, etc. all are now weak to some special moves. Another thing is that good moves get added every generation. Every Pokemon's movepool expands and expands, and their movesets get stronger and stronger. It was easier to wall the dragons back in Gen III where the best they could get Earthquake, Hidden Power Flying, Rock Slide, and then some special coverage moves coming off their more average Sp. Atk stats. But it's harder now when they're all throwing around stuff like Outrage, Dragon Claw, elemental punches, Aqua Tail, etc. from their enourmous attack stats. Not to mention back then they'd often have to set up with Dragon Dance, but that's less necessary now, they have the power and speed (or Choice Scarf) to just attack from the get-go more often.