Two thoughts:
First, why would we grant this premise? If other hypotheses about the sources on jesus are expected to be supported by evidence if we are to accept them, why would we accept this one “unless we can find evidence otherwise”?
Secondly, if we do grant the premise, what does it tell us about the historicity of the stories?
At first glance, it tells us nothing. To the extent that the stories are historical, they are indeed all derived from a single source - namely, the historical reality. So the single-source premise is not inconsistent with historicity.
In fact, if anything, it may lean slightly in favour of historicity. Consider: If there is a single source it is either (a) the historical reality, or (b) a basically fictional now-lost text on which all the others draw.
The problem with (b) is that, if such a text existed, the later authors drew on it by discarding much of it. Paul on the one hand, and the synoptics on the other, have practically no material in common. I don’t just mean that they have no texts in common; that they don’t quote each other. I mean that they don’t even deal with the same things. So, for example, if they are all drawing on the one text, then Paul is discarding huge, huge chunks of it and basing his writings only on selected extracts, while the synoptic authors are discarding different huge chunks, and basing what they write on entirely different selected extracts.
And this seems surprising. Usually, when people draw on a text to produce a new text, they add their own material to what they have taken from the earlier text. It’s on this basis, for example, that we take Matthew and Luke to be later than Mark. It defies common sense to suggest that Mark adapted Matthew by taking Matthew’s text and ditching much of it. Why would he do this? And why would a text produced in this way have any traction with a readership? I don’t think the Reader’s Digest condensed novel had been invented in the first century and, even if it had, there is no explanation for why it would eclipse the source text. And if that is how scholarship deals with the dating of Matthew, Mark and Luke, why would we assume “unless we can find evidence otherwise” that precisely the reverse process happened when the Paul and the synoptics used our hypothetical text as their source?
And even if we overlook that problem, we still have to explain why the selections from the original text that Paul and the synoptics used are mutually exclusive. Was this an amazing coincidence? Were the
In short, if we grant the single-source premise, the hypothesis that the single source was a now-lost, basically fictional text requires assumptions and presents problems which the alternative hypothesis, that the single source is the historical reality, does not present. Hence, the principle of parsimony would steer us towards the latter hypothesis as being the more likely. And, if we must choose one explanation “unless we can find evidence otherwise”, this looks like the one we would choose.