Thank you for sharing your thoughts on this subject, MATZsemantics; coincidentally, I have similar thoughts (and fears) regarding "life after death" (or in your own words, "metempsychosis").
According to current cosmology, the universe is estimated to be approximately 13 billion years old (
https://en.wikipedia.org/wiki/Age_of_the_universe), and life on Earth is estimated to have arisen 3 to 4 billion years ago (
https://en.wikipedia.org/wiki/Earliest_known_life_forms). Now, I believe it to be uncontroversial to say that these are both extensive periods of time, especially when compared to our average life expectancy as human beings. When we consider that, from our current perspective as conscious beings, all these billions of years passed by like the blink of an eye (in other words, from "our past unconsciousness" as non-living matter to "our present consciousness" as living matter), what kind of safeguard of guarantees have we - sufferers of life - that, after this present flow of consciousness ends, "we" shall not be immediately transferred to another flow of consciousness as another being (in fact, from this perspective, this same process might have already been repeating itself time and time again)? Think, for example, of open individualism (
https://en.wikipedia.org/wiki/Open_individualism).
To quote what you wrote, which I am in full agreement with: "
In other words, upon death (or birth), the subjectivity alotted similarly to a human may "pass through" the experience of countless inanimate unsubjective systems, only to experience no subjectivity and be "transferred" horizontally to a subjective being."
(By the way, when you state that, in your view, even non-sentient systems (to my understanding, non-sentient or non-living matter) also possess experiences ("nous" or mind), but that they are not integrated into a subjectivity, do you mean this in the sense that non-living matter - in an identical manner to living matter - is also constantly under physical and/or chemical changes (either in macroscopic or microscopic scales), but that, since they possess no subjectivity with which to "integrate" or "internalize" these changes as an individual and separate being from the other matter or phenomena that surrounds them (through the help of memory, of course), these non-sentient systems therefore have no "sense of self" like we seem to have at this instant?)
Anyway, going back to my point.
Let me attempt to be clearer. By definition, "non-existence" or "unconsciousness" cannot be experienced; if they could be experienced, we would refer to these states by their opposite names, that is, "existence" or "consciousness". Now, can we say, of what cannot be experienced, that it exists in a phenomenological (in other words, in an experiential) sense? I am convinced that we cannot (once again, by the simple definition of these words). In this sense (and
terrifyingly so) "consciousness", "sentience", "perception" or "life" seems to be - and I am aware that this might sound non-intuitive and even paradoxical - the default state, as their opposites, that is, "death", "non-existence", "unconsciousness", etc., cannot, by definition, be experienced, and thus exist in a phenomenological sense (in the same sense that we did not and could not experience the billions of years which came before our current flow of consciousness).
In this perspective, we may also remind ourselves of the anthropic principle (
https://en.wikipedia.org/wiki/Anthropic_principle): "
Proponents of the anthropic principle argue that it explains why this universe has the age and the fundamental physical constants necessary to accommodate conscious life, since if either had been different, we would not have been around to make observations."
Therefore, from a similar point of view to the one proposed by the anthropic principle, it is also not surprising (in fact, it may even seem trivial when one analyzes it carefully) that we are here, at this current instant, not only conscious (and making observations) but also able to exchange ideas regarding consciousness itself; for if "we" were non-existent (or unconscious), or if "we" were a kind of being whose organic or cognitive faculties did not allow for such a kind of communication as the one we are now having, we would thus, by definition, be incapable of being and communicating as we now are. To quote once again from your post: "
it's not like you aren't experiencing the subjectivity of a chair, it's that you can't".
Socrates, in this exchange with Cebes in the Platonic dialogue called "Phaedo", also seems to express a similar idea regarding metempsychosis:
"
Well then, is there an opposite to living, as sleeping is the opposite of
being awake?
Quite so, he said.
What is it?
Being dead, he said.
Therefore, if these are opposites, they come to be from one another, and
there are two processes of generation between the two?
Of course.
I will tell you, said Socrates, one of the two pairs I was just talking
about, the pair itself and the two processes, and you will tell me the other.
I mean, to sleep and to be awake; to be awake comes from sleeping, and
to sleep comes from being awake. Of the two processes one is going to
sleep, the other is waking up. Do you accept that, or not?
Certainly.
You tell me in the same way about life and death. Do you not say that
to be dead is the opposite of being alive?
I do.
And they come to be from one another?
Yes.
What comes to be from being alive?
Being dead.
And what comes to be from being dead?
One must agree that it is being alive.
Then, Cebes, living creatures and things come to be from the dead?
So it appears, he said."
Now, one could object that the heat death of the universe, also known as the "Big Freeze" (
https://en.wikipedia.org/wiki/Heat_death_of_the_universe), which currently seems to be the most accepted theory regarding the ultimate fate of the universe, and which implies that, after reaching maximum entropy (or thermodynamic equilibrium), no further changes or actions will be possible (
https://scienceline.org/2022/02/the-icy-fate-of-the-universe/), could be our "salvation", as it would seem to eventually prevent the universe from further generating organisms like ourselves who are vulnerable to a multitude of undesirable states of being (and consequently, there would be no further "rerouting" of consciousness as you mentioned).
However, our scientific understanding of the universe is, as we all know, dynamic - it is always subject to changes and revisions. Therefore, it would seem that we cannot deny with certainty that something like the Big Crunch (
https://en.wikipedia.org/wiki/Big_Crunch) might occur instead, which would potentially allow "suffering" as we know it (that is, undesirable states of being) to continue forever.
I don't know. Perhaps this universe is, similar to how the gnostics described it, ruled by a demiurge, the archons, or some other kind of malevolent entity. That is, at least, a possible explanation (even if it lacks empirical evidence to support it) as to why so much suffering has existed and continues to exist
just on this planet and/or the observable cosmos.