People stick to an animal-based diet longer than a plant-based one, so it's the better diet to recommend
Trials comparing popular diets find people drop out of all of them at similar rates, strict plant-based diets included. Adherence is a real concern, but it is a reason to help people stick to a diet, not proof that one diet is better. On health, even people who make this argument usually agree that plant-based comes out ahead when adherence is equal.
The best diet on paper is useless if people can't keep to it. In my experience, clients stay on an animal-based way of eating longer because it is filling, simple and familiar, while plant-based plans tend to fall apart. A diet someone actually sustains will do more for their health than a better one they quit after three months.
The evidence doesn't show a reliable adherence gap in favour of animal-based diets. When trials have put people on different named diets, such as Atkins, Zone, Weight Watchers and a strict very-low-fat vegetarian plan, dropout was high across the board and broadly similar between them. Every restrictive diet is hard to keep. A plant-based diet does not stand out as uniquely hard.
A coach's sense that clients stay on one diet longer is worth something, but it is shaped by who comes to that coach and what the coach believes in. A practitioner who likes animal-based eating will explain it better, troubleshoot it better and attract clients who already want it. That says more about the fit between coach and client than about the diet.
Adherence also answers a different question from the one that matters. The gym has terrible adherence, and nobody concludes that exercise is bad advice. The right response to a diet that is hard to keep is to make it easier: good recipes, familiar meals, fortified foods, a B12 supplement. And when adherence is set aside, the health comparison favours plant-based eating, a point people who raise this argument often concede.
Finally, the case for veganism was never only about which diet a client will stick to. If a plant-based diet is adequate, and the major dietetic bodies say a well-planned one is, then ease of adherence is a practical cost to manage. It is not a reason for an animal to be killed for the meal.