No, there are a few cases where people can't thrive on a vegan diet - a few specific and rare medical conditions.

Check the definition of veganism please: "Veganism is a way of living which seeks to exclude, as far as is possible and practicable, all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose." Note the "as far as is possible and practicable". In the case of those few individuals who need to consume animal protein, they would still be living vegan lifestyles if they were only consuming what they needed to, and were making the effort to live vegan as far as practically possible. It's not about purity.

Most people don't need meat, therefore it is needless, therefore it is not a compassionate way to live. The many thriving long-term vegans are proof that most people don't need meat, and besides: the the American and British Dietetic Associations have categorically stated that a balanced vegan, plant-based diet is nutritionally adequate, healthy and safe for all stages of life including pregnancy and lactation, and the UN is currently recommending a global shift towards a plant-based diet, after issuing a report comprising of the work of dozens of scientists. Do you believe you know better than these people?

I do not need mussels, oysters or other bivalves. The only thing I need is to make sure to consume enough B12-fortified foods such as plant milks and nutritional yeast, along with a once daily B12 supplement. That's literally it.