This impulse to link Mark to an eyewitness, is telling. I think we (the Royal Christian we) tend to see the Christian movement, so to speak, as starting out as its most perfection to as Jesus made it.
Isn't that the standard we're seeking to measure from, to determine if and when Christianity became poor?
But that state of Christianity, the earliest stage, or state, of Christianity, is hard to get at ; so it's been used by "Christian" con men time and time again -- The Recovery, for instance -- down thru the ages.
So it's just easier to believe and conclude that the gospels are speaking of eyewitness accounts, to what Jesus did and said, and we're getting the true Jesus, that founded the purest form of Christian teachings.
This leads to an effort to be that pure form of Christianity. The RCC, for example, claims to go back to Peter. The Baptists cooked up
Landmarkism, that traces the Baptist Christians back thru a true ribbon down to their founder, Jesus. And we all know of The Recovery.
This idea that the Christian movement strayed from it's purest form, based upon its founder, is an easy sell. And we (the Royal we again) buy it because it sounds good. Of course we want what Jesus founded.
The problem is that that earliest model of Christianity, straight from the founder, was to be a Jewish Christian.
The Hellenist Jewish Christians fixed that. Or broke it. We haven't determined which yet.