A historical reconstruction involving John the Baptist may be plausible, but I don't think it can ever become more than one theoretical possibility among other possibilities.
The main reason is that the historicity of John the Baptist is not a bedrock fact but a possibility.
The appearance of John the Baptist in the gospels serves a theological function, not a historical one. He is the prophetic go-between standing at the end of the Mosaic cult (Old Testament) and the new order (New Testament).
His death is depicted as a foil to that of Jesus so this, too, serves a theological function.
Serving a theological or narrative function does not prove that a figure was fictional, but it does offer us a satisfactory reason for believing the person was created for the narrative. We don't need to look for other reasons if they are not necessary. Example, Judas was created to fill the need for a betrayer; Joseph of Arimathea was created to fill the need for someone to bury Jesus in a tomb. These people may also have been historical but we have no way of proving that, and no reason to believe they were historical.
If it is objected that Pilate likewise has a non-historical role given that he fulfills the theological necessity of crucifying Jesus, then it must be conceded that Pilate in the gospels is indeed nonhistorical. He is not like the historical character at all. The gospels are not evidence for the historicity of Pilate. Pilate is known to have been historical on the basis of other evidence.
The only other early evidence for John the Baptist is in Josephus but this passage is problematic for several reasons that cast some doubt on its historical reliability. William Walker sets out six criteria that can point towards the possibility of interpolation and six of those six apply to J's JtB passage. Criteria are not laws that prove interpolation but they do justify doubts. Given what we know of the abundant prevalence of interpolations in ancient texts, even non-Christian ones, it is reasonable to be open to not just the possibility but the likelihood of interpolations in any ancient work we study. Ancient scholars were well aware of this problem and themselves sought to establish ways to identify them.
Criterion 1, Contextual evidence:
- The JtB passage interrupts an otherwise smooth flow of text
Criterion 2, Comparative evidence:
- In the John the Baptist paragraph the reason Herod’s army was defeated by Aretas was because God was punishing him for his unjust treatment of John. But that’s not the view of Josephus a few paragraphs later where the reason was to do with listening to silly advice from a woman
- Josephus is silent on John at a similar place in his other work
Criterion 3, Text-critical evidence:
- the passages does not appear in the earlier Greek version’s table of contents but does appear in the later Latin version.
Criterion 4, Ideational evidence:
- The tone of the passage is positive and supportive towards John which is inconsistent with Josephus, the fierce opponent of anyone seeking to challenge the legitimate government or promote rebellion of any sort"
- The epithet "Baptist" is a Christian term confined to the Synoptic Gospels (not even used in Acts or GJohn) and left unexplained for Josephus's Greek and Roman audiences; the terms Josephus usually uses when speaking of ritual immersion to for purification are λούεσθαι and ἀπολούεσθαι. "Baptist" only appears in relation to John.
Criterion 5, Motivational evidence:
- John’s outspoken dismissal of baptism ‘to gain pardon from whatever sins’, before commending his own version suggests an underlying polemic against Christian orthodox baptism. (Baptism of this nature did not exist amid mainstream Jewish circles of the Second Temple period. Such baptism appeared and developed within sectarian groups on the margins of Judaism as at Qumran. It was then carried on and practised by Jewish-Christian groups over the first centuries CE.)
Criterion 6, Locational evidence:
- Josephus had only shortly earlier said the place where Herod sent John was not even owned by Herod
So explanations for Christian origins that rely upon a role for JtB are certainly in the possible and plausible category, but I don't think they can ever be treated as conclusive.
