Eye-witness statements that are contemporaneous are not lost over time.
However here you are only taking into consideration memory issues, you are forgetting perception issues. People can perceive things incorrectly or even have their perception changed very rapidly, and when they get things wrong after just a few minutes, hours, or days, we're usually dealing with perception issues.
For example, witnesses to a car crash were all asked to estimate the speed of car at the time of the crash. Just by asking the question with a different word (smashed vs collided) the witnesses' estimates of the speed differed by around 25mph, yet all of them had seen the exact same event and the questioning was mere minutes after their seeing it occur.
Talking about the event and what was seen with colleagues, family, friends, and police, all have the potential to start shifting the perceptions of what was and was not seen. Even just thinking about it and comtemplating what happened can allow the brain to start "filling in the gaps" and creating false parts to the memory within a few hours.
The issue is not that we forget things and lose the memory, it's that our brains don't act as cameras and record things in a never changing manner, in fact our memories are very fluid, fragile, and malleable in ways that we still don't often understand and are often just coming to grips with. This is one reason that police now days have to be so incredibly careful interveiwing people, because it is so easy to create false memories and overide the real stuff by asking the wrong type of questions that lead the brain to start filling in the gaps.
And when virtually all 20 of the witnesses at a scene report the same observations, the veracity is substantial.
Based on certain assumptions. First we have the issue with perception and memory. Second we have the issue with interpretation and reporting. Witness testimony is seldom perfectly clear, and the same is true of this case. Many of the witnesses who claimed that the "back" of Kennedy's head was missing, when asked to draw or show where the wound was, place that wound above slightly behind the right ear. That is to the back of the head, but it is certainly not the very back of the head, which many Conspriacy writers try to claim, and is consistant with a shot from the rear and above, as seen in the Z. footage.
This is why Eyewitness testimony is so poor. Even when it is give first hand there are serious issues with it, when it is given second hand with the advantage of spin and selective interpretation, it is beyond useless.
