This is what Mundus is all about. It's why Lorkhan created it in elven myths (with the Dunmer adding "it's a test" to the whole thing). Even CHIM is to exist in a constant state of flux (with a high chance of Zero Sum).
CHIM is not the goal, it is stepping stone.
Yeah, but then the Time Dragon pops up, and according to that, things stop fading away. AFAIK, both Aedra and Daedra are of the et'ada that sprang into being with the Time Dragon. None of them have ever died. The "ideas" that came before them ebbed and flowed and faded away, but they themselves have never been shown to be killable. All that this quote from Sithis proves is that, if you got rid of the TD's influence, then things might be killable, but that's a paradox. In order to kill an et'ada, you first need to kill Auri-El, who is an Et'Ada.
Of course they haven't been shown to be killable after the point where they were rendered unkillable, which is why I suggest the way to kill them is to remove the mechanism that makes them unkillable (since them being unkillable only applies to this post-stabilization context). There were things like them before the advent of time, and those things died. The only difference between the Daedra and those things that died is that the former is what we call them when the universe reached the point that they were no longer dying. Reverse that state and the daedra will become what they were before, ideas ebbing and flowing and fading away, dying.
Perhaps we'd best do with another source:
"These ideas ebbed and flowed and faded away and this is how it should have been. One idea, however, became jealous and did not want to die; like the stasis, he wanted to last.--Sithis
"For ages the etada grew and shaped and destroyed each other and destroyed each other’s creations.... Here were the etada with their magic and their voids and everything in between and he yearned for the return to flux but at the same time he could not bear to lose his identity."--Vehk's Teachings
Note, the bit from Vehk's Teachings is
after Akatosh is already on the scene. With these two texts we're offered two different types of 'death'. One of them is a 'return to the flux', where 'death' results in a loss of identity (in like form as the dreamsleeve), things die in the ebbing and flowing and fading away. The presence of Time puts a stop to this. But then even after the presence of time, there are still etada destroying each other - so, they can still die.
What is it that happens next and seemingly renders some immortal? The creation of Mundus.
By this there should be two ways of killing them. One is to destroy Time, which will 'kill' everything. The other is to destroy Mundus, which will revert them back to the state where they were previously destroying each other (and really, that makes one wonder, why would the creation of Mundus cause those not involved in it's creation to be no longer destructible?)
On a random note, the term "et'ada" seems rather poorly defined, and there is conflict even within unified texts as to what it refers to. Vehk's Teachings and the intro to the Monomyth posit all spirits before the creation of Mundus as Et'ada; however, the "Myth of Aurbis" and the "Altmeri Heart of the World" attribute the term only to those who created the world:
-"The magical beings, then, having died, became the et'Ada... The Daedra were created at this time also..."
-"So they created the Mundus, where their own aspects might live, and became the et'ada."--The Monomyth
The only thing I might posit regarding it is that Akatosh/Auriel is decidedly not an et'ada, especially in the Altmeri/Vehkian scheme of things. In the broadest definition, the etada are the beings who were spawned in the flux; Auriel, on the other hand, is what allows the et'ada to form:
"Each gave birth to their souls, Auriel and Sithis, and these souls regarded the Aurbis each in their own part, and from this came the etada, the original patterns."--Vehk's Teachings
Similarly in the 'Altmeri Heart of the World', Auriel is created by Anuiel, and after Auriel's creation the etada begin to spring forth. Auriel is unique amongst the spirits (as it Lorkhan arguably).