However, if we accept the premise of the quote that every opinion now accepted was once eccentric (and the implicit inference that every opinion that will be accepted in the long run is now eccentric), and your goal is to have opinions that are accepted in the long run, then it is true that you have to pick opinions that are currently eccentric (even if it is also true that most eccentric opinions, both in present and past, are crazy ones that never become accepted).
The main problem is that the premise is false. Most conventional opinions of the past are still accepted today (e.g. "clouds lead to rain", "fire is hot", et cetera).
(e.g. "clouds lead to rain", "fire is hot", et cetera).
I suspect the kind of opinion the quote is talking about is as defined here; a belief or judgment that rests on grounds insufficient to produce complete certainty. Neither “Fire is hot” nor “Clouds lead to rain” count as examples of this as most people have a fair amount of evidence on hand to back those beliefs up.
In light of this, could you please provide alternative examples of conventional opinions that were also held in the past?
Betrand Russell's Ten Commandments for teachers.
I find this to be of use not just for teachers but for rationalists in general. #8, especially, is an especially eloquent formulation of Aumann's Agreement Theorem.