Speaking for myself, veganism is a crazy idea because it means putting yourself in a position where you can't get all the nutrients you need from the food you eat and you have to take supplements to make up for the deficiencies, otherwise you risk suffering from malnourishment.

I don't get this kind of choice at all. Nourishment should come from food, not from supplements. If you don't eat well enough to keep yourself healthy and fullfill your needs, you should change your diet. If there are ideological reasons, religious or political reasons, then you should re-examine those. There is nothing ethical in starving yourself. You will not care for animals and will not save the environment by not eating enough food.

What's more, being vegan achieves exactly 0 change to the suffering of animals and to the damage done to the environment. When you go to the supermarket and find a steak on a shelf, whether you buy it or not makes no difference to the suffering of the animal or the amount of greenhouse gasses produced to raise it. You can choose to buy the steak and eat it or not, but you can't change what has already happened. So whether you eat meat or not is just an empty, symbolic gesture. It may give you the warm moral fuzzies, but you haven't changed anything in reality.

The idea of course is that if everyone becomes vegan, then meat production will stop or decrease. It could work in theory, but in practice the number of vegetarians and vegans keeps rising but meat production and consumption has been increasing steadily since the 1960's:

https://ourworldindata.org/meat-production

So what is the vegan plan? Whatever it is, it's not making a dent. Everybody has heard the vegan and vegetarian claims by now: meat is murder, it destroys the environment, you're a bad person for liking meat, the goalposts keep shifting, but meat production keeps increasing, never decreasing, no matter what.

It's not just a crazy idea, it's an idea that doesn't work.