"There must have been insistence that the pack must be crushed between two flat plates."
Either it was intentional, or somebody didn't realize that you could do things much cheaper by pinching between rollers and applying a more limited force over a longer time. I would think the result is pretty much the same.
Rollers I don't think work right if you're squeezing intact, whole vegetables. But of course, these Juicero packets don't have intact, whole vegetables in them at all (proven by the ability for people to squeeze them out by hand).
I imagine what happened is that they designed it with flat plates so that it could press whole vegetables. However this didn't work, because as you need to mill anything except soft fruit before they are pressed. They changed the composition of the produce packs so that it did work. However by doing this they'd made the packs so easy to press that a roller (or your bare hands) were sufficient. By this point they'd already designed this beautiful over-engineered solution with flat plates, so probably didn't even try using rollers at that point.
I think rollers would work. Common sugarcane juice machines easily crush a thick, hard (almost like wood) cane into dry paper-like sheets. They can easily crush vegetables, unless you try to put a whole pumpkin into it.
Either it was intentional, or somebody didn't realize that you could do things much cheaper by pinching between rollers and applying a more limited force over a longer time. I would think the result is pretty much the same.