This here is an idea I just had...

if I were a seemingly powerful ancient being who's chosen mission is to destroy life as everyone knew it and return it to the way it once was... seemingly no end of tools and supplies at my disposal but almost on my own...? Well, the first thing that I would do is recruit more like-minded individuals. Not everyone is the shattered soul of an Ascian after all, but having the echo is supposed to be a clear sign.

So, maybe he had the Echo awakened in people so that he can see which souls he should be paying attention too and then focus on them. I mean, if you think about it, if that is what he's doing, he's not lying about 'saving the world' and all that because in his mind he is saving this world. Not only that, but apart from Lahabrea and Emet-Selch, the other Ascians were all 'incomplete' and therefore the rest of their sundered souls are still out there, right? Sure they won't be the same people we've fought before, but I can see them coming back.