That's the line of historicists like Bart Ehrman who multiply sources out of thin air. For all we know, the main source of the gospels (besides the Septuagint and pagan myths) is just Mark. The author of Mark could have made up the Jesus narrative. Positing an oral tradition or prior, nonextant textual sources begs the question of historicity. At any rate, that's what mythicists like Richard Carrier argues.

You think it would have been hard to sustain a myth in the first century Judea, after Jerusalem was destroyed and the Jews were scattered? How much fake news is spread today? How many people believe the conspiracy theories and ghost stories and corporate propaganda? And that's in the digital age in which there's a permanent, easily accessed record of everything. Two thousand years ago it would have been FAR EASIER to get away with lies, confusions, distortions, slander, gossip, or frauds of all kinds. That's just commonsense.