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.
Those are religious issues, and there is no objective standard to what we will be horrified by. The only safe bet is that humans will be different in different cultures, and thus different people will find different things bad.
People weren't horrified about cruelty against slaves not so long ago, and look at where we are now. I bet most people of that era would not believe we'd ever reach the point we got now. I bet many would even be willing to die on wars for over their now-completely-unacceptable ethic standards.
That is a pretty bad example as what we have today is not much better, it's just hidden in poor countries. "Being horrified" is something most say they are and then they go buy clothes made by people living a life as bad as slaves used to.