I don't actually think that an unconditional victory on the OotS's part is the best possible ending. For one, I'm actually rooting for Redcloak. Undoubtedly evil though he is, the resolution of his character arc is the one I look forward to the most and while I have accepted to a certain degree that it cannot end happily, I hope it will at least satisfactorily.

And while I love Tarquin, I do want to see him defeated. He's a lot less defensible that Redcloak, since he wants is personal power and glory at the expense of everyone else in the world. Redcloak at least strives to serve a greater purpose. How effectively, efficiently or sincerely, is up for debate.