Surely the net value of having the option depends on the magnitude of the chance that I will, given the option, choose suicide in situations where the expected value of my remaining span of life is positive?

Well -- here's the thing. These sorts of scenarios are relatively easy to safeguard against. For example (addressing the 'wrong button' but also the transient emotional state): require the suicide mechanism to take two thousand years to implement from initiation to irreversibility. Another example -- tailored the other way -- since we're already talking about altering/substituting physiology drastically, given that psychological states depend on physiological states, it would stand to reason that any being capable of engineering immortality could also engineer cognitive states.

I would find the notion of pressing the wrong button accidentally for two thousand years to be ... possible at all, given human timeframes. Especially if the suicide mechanism is tied to intentionality. Could you even imagine actively desiring to die for that long a period, short of something akin to an AIMS scenario?

Also, please note that I later described "AIMS" as "total anti-utility that endures for a duration outlasting the remaining lifespan of the universe". So what I'm talking about is a situation where you have a justified true belief that the remaining span of your existence will not merely not have positive value but rather specifically will consist solely of negative value.

(For me, the concept of "absolute suffering" -- that is, a suffering greater than which I cannot conceive (That's kinda got echoes of the Ontological but in this case it's not fallacious since I'm not using that as an argument in favor of its instantiation rather than definition) -- is not sufficient to induce zero-value/utility let alone negative-value/utility. Suffering that "serves a purpose" is acceptable to me; so even an 'eternity' of 'absolute suffering' wouldn't necessarily be AIMS for me, under these terms.)

The point is: such a scenario -- total anti-utility from a given point forward -- has a negligible but non-zero chance of occurring. And over a period of 10^100 years, that raises such a negligible percentage individual instance occurance to an accumulatedly significant one. IF we humans manage to manipulate M-Theory to bypass the closed-system state of the universe and thereby, furthermore, stave off the heat death of the universe indefinitely, then ... well, I know that my mind cannot properly grok the sort of scenario we'd then be discussing.

Still -- given the scenario-options of possibly fucking it up and dying early, or being unable to escape a scenario of unending anti-utility... I choose the former. Then again, If I were given the choice of increasing my g quotient and IQ quotient (or, rather, whatever actual cognitive function those scores are meant to model) twofold for ten years at the price of dying after those ten... I think, right now, I would take it.

So I guess that says something about my underlying beliefs in this discussion, as well.