Vegan diets generally have protein and other nutrients.
> why I should exchange a well-functioning animal-based diet into a vegan one where I need to supplement by default to fulfill human needs
applies also in the protein domain, seeing how vegan protein sources lack or have minimal concentrations of some essential amino acids, for instance
This is so easy you're probably doing it already even as a meat eater: Mexican chilli with rice? Beans and rice compensate for each other's weaknesses. Peanut butter on bread? Peanuts and wheat cover each other.
There are some things that are technically complete by themselves, but mycoprotein (Quorn) which I think is complete, isn't available for purchase everywhere.
Conversely if you do single-foodsource-diet with meat, you can also get problems, most famously protein poisoning and scurvy.
Veganism is a lifestyle based around the notion of reducing as much harm to animals a feasibly possible. No one claims that a vegan diet is centred around health. After all, you could be vegan and eat nothing but Oreo’s and Dorito’s.