Soylent is the single saddest thing i've ever heard of. eating is fun. cooking is fun. treating my body like a truck and pumping myself full of gas is just sad.
I love cooking, am good at it, and I think I have quite a refined taste, but what tastes good isn't often the most healthy option.
I fully agree that life should be enjoyable, but I'm not sure that every aspect has to.
I like your truck analogy. Isn't the purpose of the truck, or rather what it does best, driving at its full capacity? That's why you only pump it full of gas, and not tequila or cream.
I'm currently in worse shape than I was a few years ago, while eating more "fun". But in all honesty I got much more enjoyment out of being more fit, less tired, stronger, clearer head, and frankly looking in the mirror too.
I'll take any of that enjoyment over fleeting meals. If I could have both, great, but if not, self restraint and delayed gratification is a reward in itself.
Disclaimer: This isn't specifically about Soylent, which I have yet to try, but about the concept of eating less exciting but healthy foods.