Indeed, there is reason to think that because of the strong historical anchoring of Christianity, the religion has lost some of its universality.

This is a fairly cogently argued point in Huxley's The Perennial Philosophy.

In religions where the main characters have an almost fictional, non-historically-based story, the fact that principles are universal tends to carry on longer.

Christianity, because it has come to insist on Jesus as a figure in a specific point of time can seem to get "bogged down" in the specific aspects of his life as a human man.

While all these religious figures likely had a basis in true history (as you point out for Arthur) the point of anyone who teaches the Way, the Truth, who points towards the Atman -- is teaching a universal and timeless Truth.

To become too tied to history is to lose God for the sake of the Godhead.