We should consider a point about the end of _FHYA_ I was neglecting: the deal to cover Tortaalik's abdication did not envisage a substitute Phoenix. As best I recall, there was no particular need for a Dragon emperor at the time, just a competent one. Yet the options discussed by Aerich and Adron do not include setting a reasonable Phoenix on the throne. I'd say the deepest point of the book - the tragedy of Adron forced to take the throne vs the ambition of Adron recklessly taking the throne - is premised on the single-e theory. Also note that it is the fate of [non-great-Cycle] Phoenix emperors to grow decadent and be replaced by Dragon emperors, no mention of reasoned substitutes. I think this view is present in the Vladiad, ditto no mention.