One of the most common intellectual mistakes!
Do note however that it is an efficient mistake for many people to commit, and that is part of why it is so common.
“Once-and-for-allism” occurs when people decide that they wish to stop worrying about an issue at the margin. They might either dismiss the issue, or they might blow up its importance but regard the issue as hopeless and undeserving of further consideration. Either way, they seek to avoid the hovering sense of “I’ve still got to devote time and energy to figuring this out.” They prefer “I am now done with this issue, once and for all!” Thus the name of the syndrome.
I see once-and-for-allism with so many issues, but one recent example would be the forthcoming path of Covid and Long Covid. Most people just don’t want to think about it any more, and so they settle on something (“it’s just a cold!” or “it will bankrupt the nation!”) rather than having to do lots of intellectual revisions based on the stream of new data.
Other examples of topics that attract once-and-for-all thinking would be crypto, demographic decline, long-run fiscal solvency, various foreign policy crises, biodiversity, AI issues, the Repugnant Conclusion and Non-Identity Problems, whether we are living in a simulation, UFOs, abortion, what is the person’s ultimate normative standard, and much more.
People won’t let these topics take up too much of their mind space. But neither can they do the Bayesian detachment thing, and so they shunt these topics into settled categories and put them aside.
If you are trying to figure out a thinker and his or her defects, see if you can spot that person’s “once-and-for-all” moves. There will be plenty of them.