Earlier quoted context omitted.
That common opinion is odd, in hindsight. Natural selection ought to quickly do away with an organ that served no purpose but to occasionally kill one of the luckless organisms that possessed it. It should have been obvious that it was doing something else, or more specifically, that the gene(s) for "having an appendix" were.
Did you miss the part about it being helpful for restoring GI function after a bad disease? Things like dysentery and cholera were pretty common before modern times. An organism that can recover from diseases like this is going to live longer and pass on its genes more often. The downside of it occasionally causing appendicitis and killing the organism is likely a small risk in comparison: how often did people in pre…
What I meant was that - in hindsight - the fact that it had some beneficial function like restoring GI function should have been obvious, because otherwise, it would have been selected away.
That is, even a layperson _should_ have looked at the situation and thought "I bet it does something for us". But that's the benefit of hindsight; most (including myself) didn't. We just accepted the folk wisdom that it was useless.