Of course there is well-written fan fiction. I'm not saying that everyone who writes fan fiction is automatically a talentless hack (although the vast majority are); I'm merely suggesting that those who are talented enough to write quality fan fiction are squandering their talent in doing so. I generally regard exploring someone else's world as a waste of one's own ability. You even said it yourself: many authors start off with fan fiction. The important part of that is that fan fiction is where they start but it is not where they end.