It depends on your definition of legitimate.
Sure there are people who out there who'll deliver, but from the festivals perspective that doesn't mean they're legitimate. By the terms that the tickets are sold under - anyone charging above face value for the tickets automatically isn't considered legitimate as they're breaking the conditions of supply. Hence why all the off-site accommodation providers that have an allocation of tickets will charge face value for the tickets, and then some insane price for 5 nights in a broken down caravan.
Sorry to disappoint, but Korn are VERY unlikely imho. They are an expensive head and RfP is still smaller than Nova, not mentioning Download. Also, as was pointed out before, Korn would be another heavy name and they try to keep headliners diverse.
Personally, I would love Korn but I don't see that unfortunately.
I'd say a major reason was that both festivals were mainly marketed towards the Hungarian population (which is not the case for sziget), and it kinda made the lineups repetitive. The increasing prices didn't help either.