It's not "plausible" if he never existed. And what we are first trying to decide here, is whether or not there is any evidence for that existence. So far, despite all these pages, no evidence at all has been produced.
Yes it is plausible whether it happened or not. However I may not have made myself sufficiently clear, and I will strive to explain my meaning better.
It has been urged against the historicity of Jesus that there are no sources pre Mark on the execution of Jesus by Pilate. Or at least none can be cited by name as an extant written work. I leave out the controversy about Paul, for the sake of simplicity. Now, I contend the following points
- If there was an HJ, he was crucified by Pilate. That is one of the necessary attributes of such a person. Otherwise, there was no HJ.
- It is plausible that Jesus was executed by Pilate, since Pilate was a murderous tyrant given to the practice of executing people. And if Jesus had done and said even a few of the things attributed to him, Pilate would have whacked him without the least doubt.
- A plausible story is not necessarily a true story. But an implausible story is unlikely to be a true story.
- The absence (leaving Paul aside again) of an extant pre Markan text relating Jesus' execution is not fatal to the idea that Jesus was a victim of Pilate. Such a simple fact as that Jesus died in that way would have been noted by his followers on the occasion of its occurrence, and could most easily have been transmitted for a few decades, either orally or in casual texts now lost. It is simple, striking and - to followers of Jesus - important. Its absence from the surviving public record would merely reflect the fact that this event was of less importance to people who were not Jesus' followers.
So to the question: why did Jesus' later followers believe he had been executed by Pilate? I think the most plausible (see definition of this term above) answer is that it really happened. That is not of course the most plausible explanation, by contrast, of why some of them believed he had been born to a virgin mother. Now, if indeed Paul also said Jesus had been executed by Pilate, why there is the very pre Markan text we have been looking for!
Contra the "strong" mythicists, I don't accept that Paul or Philo or anyone else believed Jesus was a purely spiritual figure. These mythicists have made no good case that anyone thought this in reality. Moreover Jesus looks not like a mythical figure, but like a peripatetic apocalypticist, of whom we know there were many at the time. To his name and deeds, if he existed, have been added many mythical features, incorporated into the beliefs surrounding him. But I stress that this is probability, and not proof that there was an HJ. My point to you is that the supposition of a non recorded pre Markan source for the story of the crucifixion doesn't seriously militate against the probability of its truth.