Luckily most plants contain a variety of those essential amino acids, with some plants containing a complete protein profile. Making it pretty hard not to get the right balance of amino acids on a varied whole foods plant based diet [1].
Vegans supplement B12 and omega-3, not amino acids. Those supplements are also fed to the animals that are supposedly providing the nutrients “naturally” [2, 3]. So it’s more efficient to just get them directly without the suffering middle man.
It’s not arbitrary to exclude plants from moral worth. They don’t have the neural substrate to deploy conciousness, animals do.
Possibly even longer, people biking might switch to using cars and more than 2 car lanes doesn’t do shit.