That does seem odd. it only took a mere 20 years to see the potential of the hero's from D3 so add another and there should be some insanely prodigious new generations too boot. But perhaps it takes time to see and manifest that lvl of power. I know that the ghosts for every hero in RoS basically said that generation had it and just needed time and mentoring to really bring it out with the sorceress as the example had she lived she may have seen Li Ming really be unrivaled as the greatest of mages/wizard/etc.