Legumes to me are healthy, but i can't see them as 100% peak human food, you have to soak them for hours and many people get bloated, then there are also certain amounts of anti-nutrients.
I’m expecting truly engineered food to be a breakthrough over the next 100 years. Imagine people being able to eat something akin to a chocolate bar that has all the nutrients one would expect from a full meal. Protein shakes are close to this, but they are still just a mix of vegetables mostly (and the ones I drink have high sodium.) I know everyone is expecting lab grown meat, and I’m sure there will be some of that, but I think we aren’t thinking about the possibilities and it could be people in 300 years eat textures and flavors far better than anything we can imagine, while being as healthy as physically possible.
I think this is realistic, in spite of the opposition we can expect from both Big Ag and Big Fake Yoga.