Words of wisdom.
I'm not sure we need a whole thread or even a poll option to demonstrate that bass failures are extremely rare occurrences in a relative sense; even people who have experienced a failure would readily admit that. It's not even debatable. It's factually extremely rare for a well-maintained bass to fail.
The point is in your bolded section above. Some people can shrug off a bass failure, which I define as anything that makes you adjust, lose time, or play and perform in a different way than usual. So I see a string breaking and a strap button popping off as a "failure," which includes even small things that would impede my ability to perform at 100% uninterrupted from start to finish. But everyone's situation is different, and I know a lot of musicians who wouldn't be bothered by these "failures" at all. More importantly, I know a lot of musicians who do a lot of performances where no one, audience, band mates, talent buyer, venue owner, et. al., would give a rat's ass if their bass had even a minor "failure" mid-performance. Hell, at some of these gigs the bass could drop out entirely for the whole set and no one would blink an eye over it.
Probably the reason you still concede that it's not a bad idea to have a backup bass is for the rest of us who, either from a sense of pride, or due to the strict and high expectations of our performance situations, don't really have that option (without negative consequences) to change out a string mid-performance or sit down when our strap button pops off (I'd have to take the extra step of adjusting my microphone stand) or fudge through until break with one less string.
Odds of a bass "failure" = slim, no doubt about it. Negative consequences if a "failure" does happen, for some of us = serious enough to pay the relatively small price and schlep a back up bass, and have it tuned up and at the ready in that extremely rare event of a "failure."
Let the reader decide what category they fall under.