This passage is not a blanket endorsement or rejection of the value of eyewitness testimony in general, but rather gives a criticism for not believing the specific testimony that had been given:
- By Mary Magdalene (vss. 9-11)
- By the two disciples (probably on the road to Emmaus)(vss. 12-13)
Elsewhere Jesus specifically teaches that not all testimony should be believed:
Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves. (Matthew 7:15)
This counsel is only practical if both of these statements are true:
- There will be those who bear true testimony
- There will be those who bear false testimony
To unilaterally accept or reject all (purported) eyewitness testimony is not the point here. True testimony had been given by known, faithful witnesses, in fulfilment of promises Jesus had made. The disciples were expected to take that testimony seriously because they had good reason to believe it was true.
A useful analogy: let's imagine an Israelite named Simeon who lives at the time of Moses. God gives Simeon reason to trust that Moses is a true prophet whose teachings come from God. Moses then gives prophetic guidance and Simeon is expected to hearken to it. God may not immediately give Simeon the reason for the instruction, but Simeon is still accountable to follow it. Not knowing why this specific instruction was given does not invalidate the testimony of Moses' prophetic leadership that was already given.
Back to Mark 16 - the disciples had already been given reason to believe that Jesus would rise, even if it took some time for them to grasp the magnitude of what this meant. They were scolded because when they received evidence of the fulfilment of Jesus' prophecy, the initial reaction (by some) was doubt.
Luke provides additional detail regarding the inappropriate initial reaction by some who heard the testimony of Mary Magdalene and others:
And their words seemed to them as idle tales, and they believed them not. (Luke 24:11)
Clearly the Lord saw treating Mary Magdalene's testimony as an idle tale as inappropriate. We can reasonably conclude, then, that over the course of His ministry, He had given them reason to believe both this message and this messenger when the time came.
For a deeper dive on the authorship of the last 12 verse of Mark, this video on my channel may be of interest: The Ending of the Gospel of Mark
Since all but a few words of the last 12 verses of Mark can be found in Matthew, Luke, Acts, and John, we could reconstruct the same argument used above from other New Testament texts regardless of what conclusion we reach on the authorship of the longer ending of Mark.