In hindsight, I don't know if there is actually a difference in volume of challanging RPG's across the ages.

NES days probably had the most. Get to SNES, the hardest games on that were FF IV, BoF 2, and Ogre Battle in terms of RPG's. The only real consistantly hard RPG series I can think of is Dragon Quest. Otherwise it's just the occasional difficult RPG, and bunch of moderately challanging RPG's, and some easy RPG's.