Or this:

Nathanos gets dumped directly into the Maw, but Sylvanas and the Jailer don’t really give a shit. The Jailer because he doesn’t really give a shit about people dying in general, and Sylvanas just kinda ditched Nathanos after forsaking the Horde so I guess she doesn’t care about him anymore either.

We’re going to the Maw to pull souls out of it in Shadowlands, so what if one of them is Nathanos? We pull him out of the Maw, then lock him up in our Covenant Hall and interrogate him. It’s an interesting situation because Sylvanas actually killed Nathanos’ cousin just to make Nathanos less ugly pre-Legion, and they describe his face as “not entirely his own”. So maybe it all kinda hits him at once, and he realizes how horribly he’s fucked up. We get a Punished Nathanos arc, he turns on Sylvanas. Then at the end of Shadowlands, he decides to stay dead this time.