There is certainly a solid body of peer-reviewed scientific evidence to confirm that from a dietary perspective, it benefits human health to consume a primarily plant-based diet. Eating more plant-based foods is beneficial for our health and for the environment. There's no argument about that.
However, we need to make a distinction between veganism and a plant-based diet. Surely, a completely plant-based diet is a part of being vegan, but veganism represents a larger ideology that defines a commitment to opposing the use and exploitation of animals. Vegans don't eat animals, but they also don't wear them, visit places that enslave them, or in any other way participate in the commodification of animals as much as humanly possible.
While we can say there are health benefits to adopting a plant-based diet, the truth is that there are no health arguments to avoid wearing wool, visiting zoos and circuses, or buying products that are tested on animals. Even the dietary argument only goes so far. You cannot necessarily say that someone who eats a predominantly plant-based diet, which includes small amounts of animal products, is consuming an "unhealthy" diet.
We commonly see celebrities and other high-profile people claiming to have gone "vegan," only to return to eating animal products again and then being labeled as an "ex-vegan." The fact is, they were never vegan in the first place. They were simply consuming a plant-based diet and their only commitment was to their own personal health, which is why it's that much easier to give it up. When people are committed to living a lifestyle in accordance to principles that extend beyond oneself, it makes turning back much less likely.
There's no doubt that many people have adopted a plant-based diet for health reasons and then transitioned to a vegan lifestyle after becoming more informed of the ethical implications. For that reason, it does continue to make sense to use health as a motivating factor to help people overcome the dietary aspect of transitioning to veganism. For many people, adopting a completely plant-based diet is the first step they're willing to make. Any progress in the right direction is commendable.
Although, we need to be clear from the beginning that changing dietary patterns does not define a vegan lifestyle. Many people make the argument that the only way to get people to transition to a completely vegan lifestyle is through health. There are many people who have proven that's not the case and it's also reductive to assume that people are incapable of understanding the message of compassion. So, as long as veganism is perceived as nothing more than a diet, we cannot expect it to be respected as anything more than such.