Cooking doesn't take long. At all. You can prep an entire week worth of meals in 1 hour. Refrigerate them, freeze them. We have thousands of years worth of food preparation and preservation technology. Cook on Sunday, eat until Saturday.
As for being expensive: again, it isn't. If cooking at home was expensive the world population would be 2 billion tops. Poor people cook at home because it's the only way they can survive. If a woman in Paraguay working two jobs can do it and feed her family, so can you.
> it's about providing a convenient, nutritious, and (relatively, compared to fast food) cheap source of nutrition for those times that I don't have social obligations and I don't want to/don't have time to cook something for myself
You can fix yourself a sandwich using good products that is just as nutritious. Most people don't even need to eat so much. It's very easy to have good 1500-2000 kcal a day with regular food that you prepared quickly.
Soylent, to me, is simply about people who want to "hack" their bodies and be on the futurist side of things. There's nothing nutritiously advantageous about it, nor is it cheaper than cooking for yourself. But somehow people make the assumption that it's either that or eating the very worst food known to man (McDonald's or whatever). It isn't.