There are 3 reasons for choosing a vegan diet: health, environment, and animal welfare. As you point out, much junk food is vegan, and some work is needed to make a vegan diet healthy (B12 supplements especially). Less work is necessary to maximize environmental protection on a vegan diet (how the food was grown, its impact on the environment (e.g. avoid palm oil), etc. There is no question that a plant-based diet will greatly decrease suffering (I’m ignoring what for me are trivial issues like avocados and bees, but everyone should research problems in this category as well).

Any one of these justifications is a good reason to move toward a carefully chosen vegan diet. All three combined make meat-based diets all the more questionable as a choice in 2019.