The first principle uses the word "should" but the last uses the word "would". I'm going to assume you mean "would" in both cases, since presumably what any observer *should* want is identical to what an ideal observer *would* want.
That said, we need to define what we mean by "ideal" or "perfectly moral" observer. There are two plausible definitions that I can think of. However, both definitions imply that one of the premises in the argument is a trivial, analytic truth and the other other premise is question-begging.
On one definition, an ideal observer is just an observer who wants everyone to perform right actions. In that case, premise 2 is a trivial, analytic truth whereas premise 1 is question-begging against deontology. The premise "Between any two events, ideal observers should prefer that the one that makes the world better occur" would be equivalent to "Between any two events, [observers who want everyone to perform right actions] would prefer that the one that makes the world better occur". But this alone is sufficient to imply consequentialism. Why would this premise be compelling to someone who didn't already accept consequentialism?
On another definition, an ideal observer is just an observer who wants the world to go best. In that case, premise 1 is a trivial, analytic truth whereas premise 2 is question-begging against deontology. The premise "If an ideal observer would prefer one take some action, they ought to take that action" would be equivalent to "if [an observer who wants the world to go best] would prefer one to take some action, they ought to take that action". But this alone is sufficient to imply consequentialism. Why would this premise be compelling to someone who didn't already accept consequentialism?
You point to some reasons for the premises in the later paragraphs, but they are unintelligible without defining what an "ideal" or "perfectly moral" observer mean.
The first principle uses the word "should" but the last uses the word "would". I'm going to assume you mean "would" in both cases, since presumably what any observer *should* want is identical to what an ideal observer *would* want.
That said, we need to define what we mean by "ideal" or "perfectly moral" observer. There are two plausible definitions that I can think of. However, both definitions imply that one of the premises in the argument is a trivial, analytic truth and the other other premise is question-begging.
On one definition, an ideal observer is just an observer who wants everyone to perform right actions. In that case, premise 2 is a trivial, analytic truth whereas premise 1 is question-begging against deontology. The premise "Between any two events, ideal observers should prefer that the one that makes the world better occur" would be equivalent to "Between any two events, [observers who want everyone to perform right actions] would prefer that the one that makes the world better occur". But this alone is sufficient to imply consequentialism. Why would this premise be compelling to someone who didn't already accept consequentialism?
On another definition, an ideal observer is just an observer who wants the world to go best. In that case, premise 1 is a trivial, analytic truth whereas premise 2 is question-begging against deontology. The premise "If an ideal observer would prefer one take some action, they ought to take that action" would be equivalent to "if [an observer who wants the world to go best] would prefer one to take some action, they ought to take that action". But this alone is sufficient to imply consequentialism. Why would this premise be compelling to someone who didn't already accept consequentialism?
You point to some reasons for the premises in the later paragraphs, but they are unintelligible without defining what an "ideal" or "perfectly moral" observer mean.