If people were 100% rational, would the world be better?

[LadyForCamus]

If we're homeostats... living creatures with a tendency towards a stable equilibrium - and cognition is a physiological response - then the bare minimum normative case would suggest that a living, competent, and uncompromised cognitive agent should/would register a strong selective preference for life.

Sure, but the instinct to live is just that; a biological instinct. Simply that we can be rational; a de facto attribute or characteristic of being both alive and cognitively capable; isn’t a rational argument for the continuation of life itself, or that the choice to live is somehow a more rational choice than the alternative. At least as far as I can tell. It sounds like you’re saying, “choosing life over death is the rational conclusion because if you stop living you’ll be dead.”

Which is to say that the choice (and subsequent mountain of natural behaviors) follows from premises genuinely apprehended by that agent, premises which we would grant without qualification - whereas ideation to the contrary would strongly suggest that something has gone horribly awry for that agent.

What premise? That life is preferable to death? Or, that life is “better”
than death? I’d say both of those are ripe for debate. 

Consider this, while we might easily accept any number of rational cases for life, of the relatively few rational cases for death we might accept - do we imagine the person to be thinking clearly?

I would say yes. Not in all cases of course, but yes.