I kind of assumed the people who just felt sorrow at the starshower were still possessed of Ancient souls, albeit ones not strong enough to manifest an Echo.

We know there were significant disparities in the "creative potential" of the Ancients, so perhaps souls that were exceptional in that era still possessed the requisite qualities necessary to use it post-Sundering.

I am also left wondering if they'll touch on the fact that the return of the Final Days would've logically led to the awakening of more Echo bearers in Thavnair, Corvos, and other regions where the skies burned.