The fact that Aslan’s take on Jesus is not original doesn’t mean it’s necessarily wrong. But it has the same problem that bedevils most of his competitors in the “real Jesus” industry. In the quest to make Jesus more comprehensible, it makes Christianity’s origins more mysterious.
Part of the lure of the New Testament is the complexity of its central character — the mix of gentleness and zeal, strident moralism and extraordinary compassion, the down-to-earth and the supernatural.
Most “real Jesus” efforts, though, assume that these complexities are accretions, to be whittled away to reach the historical core. Thus instead of a Jesus who contains multitudes, we get Jesus the nationalist or Jesus the apocalyptic prophet or Jesus the sage or Jesus the philosopher and so on down the list.
There’s enough gospel material to make any of these portraits credible. But they also tend to be rather, well, boring, and to raise the question of how a pedestrian figure — one zealot among many, one mystic in a Mediterranean full of them — inspired a global faith.
Actually, I would go a step further and say that the Bible itself doesn't paint a full picture, because no book (even a divinely inspired one) can do this for us. When correctly understood, it may contain all of public revelation, but this "correctly understood" is already more than we can get out of this book. Hence the variety of heresies (to say nothing of the East-West Schism and a few smaller schisms) which are formed and reformed, often claiming to be getting at "the real revelation" of Jesus. The various heresies could in one way or another find ways to interpret Scripture to mean something other than what it says--whether that twisting has to do with Christ's human nature of His divine nature, or human nature before and after the Fall.