Not going against your general argument and I don't want to derail the conversation, but saying veganism is a fad is perhaps not a good example to give.

There is solid evidence against vegan diets being a fad, unless you regard a >3% yearly sales growth of vegan-labeled food or a roughly 600% increase in google searches since 2004 a fad[1][2], in addition to the roughly 500m-1b people who are on a mostly plant-based diet for cultural or practical reasons[3]. I'd wager that only a small percentage of people are in it solely because of ethical or moral reasons.

[1] https://www.statista.com/statistics/562911/global-sales-grow... [2]https://trends.google.com/trends/explore?date=all&q=%2Fm%2F0... [3] https://foodrevolution.org/blog/vegan-statistics-global/