

I thought it was mostly a jamming type effect. The memories are often still here, but become progressively less accessible. They then degrade from lack of use.
It might well be that a lot of memories still exist for longer than it seems. Even if not, stopping the degradation, and allowing new memories to form would allow people to remain cognitive and functional for a LOT longer.











https://en.wikipedia.org/wiki/Tachyonic_antitelephone
Basically, an “instant” communication device can create paradoxes.
E.g.
Earth sends mars a instant message. Mars the relay’s it via normal radio to a passing ship/probe moving at 0.99C. That probe then sends an “Instant” message to Earth.
The final message will (depending on alignments etc) arrive before the original was sent. If the message the voids the reason for the original message to be sent then a paradox happens.
You can make similar things happen with any form of FTL communication. C isn’t technically the speed of light, it is the speed of information. It just happens that light has no other speed limiting, and so moves at maximum speed.