Interestingly, I covered this in detail in DELIVERANCE OF DRAGONS, but since you won't see that until May, I can summarize it here. What the Elves drew their power from pre-Mosirinde was other reservoirs of Light, i.e. Elves, animals, and Otherfolk. (Yeah, there was not a huge amount of difference between them and the Endarkened in those days.)
But in the post-Dragonmage era, Elves capable of becoming Lightborn are not being born. They have surrendered their magic to the Light. For a few thousand years after that surrender, there are still Dragon Mages around, but eventually Death breaks the last Bond and all there remains are ordinary Elves with "small magics" and unbonded Dragons. Since the Dragonbond does not rely on magickal adepthood a new Bond could probably be formed at any time, but the Dragons are understandably wary about doing so.
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But in the post-Dragonmage era, Elves capable of becoming Lightborn are not being born. They have surrendered their magic to the Light. For a few thousand years after that surrender, there are still Dragon Mages around, but eventually Death breaks the last Bond and all there remains are ordinary Elves with "small magics" and unbonded Dragons. Since the Dragonbond does not rely on magickal adepthood a new Bond could probably be formed at any time, but the Dragons are understandably wary about doing so.