> It would serve to optimally satisfy demand...
Well, it would serve to optimize (or come close to doing so) some utility function, which might be labeled "satisfaction of demand". But who defines that utility function? Some central planner.
But how does that central planner know what I actually want? Yeah, I could tell them with my smartphone. That's going to tell them a pretty coarse approximation to what my actual desires are, though.
For example: Life Savers candy. People choose to buy the stuff. A roll of them costs, what, $1? But how many people are going to bother listing that on their phones? A whole lot fewer people than actually buy Life Savers, I bet.
[Edit: What's actually going to happen is that an element of politics is likely to creep into the definition of the utility function. "Oh, they shouldn't want that; they should want this instead."]