There is no need for vegans to worry about protein; as you eat a wide variety of (organic) grain, beans, nuts, vegetables, greens and fruit) your own metabolism adjusts your tastes and preferences so what’s most important for your body will become the most tasty.
In all my years as a vegetarian (which is almost the same as being a carnivore because you are still consuming animal protein) my palate hardly changed from when I ate meat. My mainstay were dairy and eggs, plus some starch foods. I was not particularly interested in greens or beans or grains, etc. Now I love them, and I have even begun to cook, which I never did, all those years. And it’s not because I can’t get enough of the food I like, but because I now like so many more foods!
I think the key is animal protein: We are metabolically omnivores. We are capable of living on almost exclusively meat, if we can get it (carnivore mode). But we are also capable of living on exclusively non-meat (herbivore mode). And the biological “cue” for which mode we are in is animal protein.
I don’t think the cue is graded (i.e., I don’t think that the less animal protein you eat, the more appetite you have for vegetables): I think it’s more like an on-off switch between the two modes (which, for me, took 8 months to become perceptible): Once your body is getting no animal protein at all, the metabolic switch is set to herbivore mode, and both your appetite and your way of metabolizing what you eat changes (in my case, dramatically, because I could compare it with almost 50 years of being a vegetarian, which is just a form of carnivore).
The switch is not irreversible. We can start eating meat again and it (much more quickly) switches back. I think this has to do with our evolutionary history: availability of food varied seasonally, climatically and geographically (and we migrated a lot): we were opportunistic omnivores, and ate what we could. Sometimes many generations (or much longer) some of our ancestral populations had to make do with no meat at all, or, as in the frozen north, on almost nothing but meat. Our metabolism is adapted for both.
It’s also adapted for opportunistic theft, rape, murder, infanticide, genocide, domination, torture and enslavement.
But that’s no excuse for doing it when you no longer have to. And we no longer have to steal, rape, murder etc. today (and certainly not in the civilized, prosperous, law-based parts of the world).
And killing sentient organisms for food is one of the things we no longer have to do in order to survive and be healthy.
So if we keep doing it, it is — as with stealing, rape and domination — just because we feel like it, because we have cultivated a taste for it — and not out of biological necessity.