Zarathustra wrote:
We can also wonder why Covenant had to accept Foul, instead of fight him. Thematically, I understand why accepting Foul is the culmination of the three-Chronicle saga, but the literal reason in the plot for why Covenant can't fight Foul has always been that using enough wild magic to kill Foul will destroy the Arch. But that logic flies out the window once the Arch is already destroyed. There's no longer any danger. One quick, ultimate blast, and he's gone, then repair the world.
I was thinking about that too. What makes sense to me is this: first we were originally told "use too much power and shatter the arch of time/wake the worm", "the worm will destroy the earth", "the worm will shatter the arch of time", which all sounds very instantaneous. But that was mainly due to our not knowing how the process works, and that it was gradual - the arch was being destroyed, not already fully destroyed. And I think this can also answer the main question in this thread. Sorry if this is going to be a somewhat long comment
After the worm was roused, Berek told us that the world is not unmade in a moment. It's a
process that takes time. The worm starts to move, it eats the elohim, its suitable initial food, and then it seems out the earthblood, its final food. And we are told that when the worm drinks the earthblood, the arch will shatter. But again, that sounds instantaneous due to our not having all the information.
Later still, the worm actually starts to feed on the earthblood. The process takes a while. Time starts to unravel around the worm (trees flicker in and out of existence in garroting deep), then shocks travel farther to the rest of the earth. Things start to come undone.
Given all that, I don't think the arch was totally destroyed at the point in time you refer to - when they could have blasted Foul. It was in the
process of being destroyed. If enough power was used to blast Foul, it might then shatter entirely and it would truly be game over. But it was just in the process of shattering. It was doomed to shatter, but still partially intact. Right before the chapter ends, the status is described as "
the accelerating collapse of Law and Time" - it's a process, not yet completed.
So blasting Foul would have unmade the world entirely, and it could not have been rebuilt. (It might have been recreated from nothingness, but not by living beings that require time to exist and act, so that option was out, Covenant made that clear when he told Roger how Foul could not take him to anywhere safe after the arch was done.)
Getting back to the main topic of this thread, I feel like the crucial point is exactly the process-like nature of the end of all things. The worm drinking the earthblood is
not a switch, it's a source of immense power that the worm consumes, becoming more and more powerful, and that power
piece by piece shatters the arch. But since it is done piece by piece, it is also possible to undo what the worm is doing,
also piece by piece.
During the last chronicles we saw ceasures being resolved out of existence, which is basically fixing of breaches in time and law. And it was specifically mentioned that Joan could eventually destroy the arch and the earth, by creating ceasure after ceasure over a very long period of time. I think that's analogous to what the worm causes when it drinks the earthblood, except that Joan as an intent-less tool could not wield enough power to do it quickly, whereas the worm can do it in less than an hour.
So in some sense the harm caused by the worm is not uniquely new or different, just much more. It's much much harder to undo the worm's harm as it causes it than to undo Joan's harm, but still possible in theory, just a matter of getting enough power to counter it, piece by piece - and the ending implies that by some combination of intent, knowledge, wild magic and law, as well as perhaps a lingering Timewarden affinity for the arch, it was achieved.
Finally, the worm does not crave the end of the world or the collapse of the arch of time. The worm hungers for specific forms of earthpower. And the legends say that it has a
finite appetite - it awakes, eats, sates, and eventually slumbers again. The bad part of course is that as it eats, it destroys everything - we were told that the worm ate all the stars in the creation myth, and no earth was left, a new earth arose around the worm in its slumber. But this time, two things were different.
First, most of the stars, the elohim, were saved by Jeremiah's Fane. Second, we have the extremely-powerful trio which can mend rents in time.
Together, that means that the immediate harm the worm causes can be repaired after it. And after it finally eats its fill, and wants to rest, it won't wander around doing more harm, since the surviving elohim can fulfill their purpose (as described earlier in the books) of helping it slumber properly. They can't put it to sleep, but once it
needs to sleep, they can direct it to a proper place and manner of slumber, which restores the One Tree and so forth.
In conclusion, I think the ending does make sense. The actual act of saving the world was, interestingly and oddly, skipped over, but I do think enough hints were present. Basically, the main twist from my perspective is this: Yes,
you can't defeat the worm - nothing can. It being roused starts a process that inevitably causes enough harm to undo the world. But
you can undo the harm, if you're powerful enough and fast enough. The last few chapters showed us how the trio became capable of that. And even though they may have assumed like us that the worm touching the earthblood is instantaneous game over, it turned out to be, like all other parts of the worm's life cycle, a gradual process. So unexpectedly, they were capable of using their power to save everything.