What are the odds of having large chunks of learned knowledge wiped out at this point? When the Romans did it, few could read/write and few people had access to the stored information of the day. That made it very easy to sequester knowledge. At this point in time, other than ELE type of events, can knowledge be kept away from all of humanity now? Sure, dictators can make it hard for the parts they control, but the rest of the world keeps on learning.