What sets the current dialogue appart from the "meta-Moë" discussion, at least for me, isn't a question of did either fail. The answer to that is irrevocably yes. Moë was blind/ingorant/dismissive about the effect the Outside could really influence the Inside and he died for that. Kellhus was dismissive of the idea that there was something that neither he, nor Ajokli could see, and he died for that. Both are failures.
What isn't clear is what happens to him, exactly, now that he did. It's hard to believe that with him knowing the metagnosis and mastering the Daimos, that his soul simply would just disappear into damnation. Especially not given how he bartered with Ajokli, is subject to the scrutiny of numerous Ciphrang, not to mention the other 100. That and his own saying how he made treaties with Hell itself (unclear if this is separate from Ajokli or not).
Not to mention the bizarre glossary entry for the Decapitants. Note, that in the episode recounted there, he is at the Plains of Mengedda, one of the biggest topos outside Golgotterath itself (if not the biggest). So, what was he doing? Perhaps testing what happens between swapping and topoi? While we don't know, I have doubts now that the head he put on Malowebi was just some "random ciphrang" now, considering that somehow the other head was Ajokli, or actually, I should say, was Kellhus?
Indeed, it seems that Malowebi's head is still alive in the end. So, I wonder if the other head, Kellhus' head is too? We just don't know what would happen to a disembodied head when it's body is gone. Perhaps that is where Kellhus soul really is?
Another reason to think that Kellhus soul is not in the Outside is how Ajokli seems to be looking for him, via Cnaiür. It seems to be a plausible reading that Ajokli thinks Kellhus is in the Sarcophagus. In any case, it seems that Kellhus somehow renegged on his deal, either by not delivering the souls of the whole Ordeal or by forfeiting his own soul. In any case, the fact that Ajokli can't find Kellhus is a clue that something additional is going on with Kellhus. That doesn't mean he will be central in the next series, but he may well factor in.
An additional crack-pot theory could be that the head he put on Malowebi was actually his own. So, Kellhus-Malowebi is now loose in Zeûm...but that is doubtful.