My argument's a bit of muddle so I'll use bullet points.
1. The way we understand things is essentially narrative with what comes before causing what comes after.
2. Conciousness is something to do with the creation of an internal narrative from all the feeds the brain is receiving from senses, memory (i.e past narratives) and reasoning (construction of new narratives).
3. Myths are archetypal instances of narratives.
4. Myths can also be seen as rules for living, or about life, but that's not to say that this is the purpose of mythology.
5. Mythology is culturally bound.
6. Being culturally bound, mythology can be affected by those who influence culture.
7. Historically, culture has been very bound by distance and social class.
8. This isolation has lead to certain myths becoming dominant within cultures.
9. With the expansion of the media, the exposure to many cultures, this isolation has been eroded.
10. Also distance in time from the origins of some of these myths has seen them be replaced with others that are more pertinent, or at least couched in a more modern idiom.
11. In fact, many modern mythologies are entirely fictional. It's not the existence of the characters and events that is important but the underlying mythology and whether this narrative resonates with the readers.
12. There's not much evidence to say that ancient mythologies aren't fictional either.
13. In fact, the power of a mythology is to a certain extent related to what it doesn't say. Anything too prescriptive leaves little room for interpretation, anything too detailed becomes open to attack on points of minutiae which are seen to invalidate the major hypothesis (a bit like the way in which our legal system operates).
14. Whilst being modern enables readers to more easily understand a myth, the age of a myth gives it credence because it establishes its credentials. "This has also been so", it says. This is partly why New Age religions are always looking to establish links to the past so they can be said to be Ages Old.