Look, I understand rarity, and incentive, and all that, but these are cool skins and lots of people people write them off because they're just unrealistic to get. 1 guaranteed Scion skin, and no chance at the others. I think this is the only content in the game like this - everything else rng is sellable.
Youcould literally do a tequatl a day from when they implemented sunless skins to the day Anet shuts the servers down and never get a drop. And that's just discouraging.
The system for raids where defeating the boss gives a chance at the weapon you want, but also drops a handful currency you can save if rng isn't in your favor is awesome.
or if Anet isn't sure about a currency, just add a drop of like a "Slightly Damaged Hoard of the Sunless/Drakkar/Wurmslyer/Scion" which is just the skins (so no stats), and is TP-able.