Assume the smoothies have the same nutrient profile as a regular diet. Like if you drank runny mashed potatoes and protein powder to simulate a normal food intake.
If you do poop because of the nutrients, is there a healthy diet at which waste products are no longer left in the intestine to turn into poop?
Drag has no intention of doing this, drag is just curious about the science.
In addition to what you said fiber is an important part of poop. An not just as stuff to push through the tract.
A important function of the liver is body detox. One way the liver does that is to package chemicals so they bind to fiber and get flushed out of the system. Lack of fiber can cause those chemicals to stick around and cause more problems.