It also wasn't so long ago that Tempering was considered to be irreversible and something that could result only in slaying the Tempered as an act of mercy. Quite a few characters suggested as much only for a cure to be found at a later date. In itself, that is but one example of an established 'impossibility' later turning out to have some workaround.

I also rather suspect that the Ancients would stand a far better chance fighting against the Final Days had they not been dealing with a subversive traitor within their midst who deliberately withheld key information that would have served to aid in the investigations.

Furthermore, when increasingly contrived plot devices such as time travel and alternate universes are embraced to justify the continued existence of major protagonists, then it stands to reason that such methods could in theory be deployed in the case of the antagonists as well.