This gets exactly to why I don't like this. There are two ways to justify a new development like this so late in a long involved story. The weak way is to say nothing that happened so far rules it out. The stronger way is to show how something that happened in the previous thousands of pages or dozens of hours of television actually supports, foreshadows or suggests it. Turning the dead into WWs seems to meet only the weak test, and that is, well, weak.
This is a story where major plot developments were foreshadowed in the first few chapters or hours, not to be paid off until much much later, and in all the pieces I just read taking the position that the dragon is a WW, not one has pointed to evidence in favor of the theory that the dead can become WWs. All rely on the weak justification, that nothing rules it out. Which is such BS because everything pointed to the dead/alive distinction, including that it just makes sense. If you're dead you lose your soul/consciousness/selfhood/whatever and so become a brainless zombie. If you're alive, the opposite. Yes. That was never explicitly proven but literally every piece of evidence we have actively points that way. That none of them explicitly rule out the opposite is just so lame.
There are 2 ways to write really long involved stories like this. One is to set limited waypoints and destinations and then make up the rest as you go along. Think Battlestar Galactica. The other is the plot out every minor detail ahead of time then just produce what you planned. Think Babylon 5, LOTR (B5 had to make adjustment due to personal issues and ratings but even those were pre-planned). I think GMMR has worked in a hybrid mode, creating a vast universe where lots of rules and plot points were set in advance, but others were left to make it up as he went along. And I fear that combination has turned into an unresolvable chaos, which is why each new book takes 5, 6, and even more years each, and why the TV show is becoming a hot mess even as it hits every major plot requirement while it rushes to its finish.
I'm still enjoying it, but it's not living up to what it should have been.