Movable solids would only be possible if the idea itself were somehow translated into TPT's mechanics, which isn't easy for the idea since treating whole blobs of material as single objects doesn't really exist yet. TheEyes made a mod that shows off moving solids but the idea isn't quite what most people mean when they say it, I imagine.
If moving solids were made. and functioned how everyone expects, tpt would be way slower than it already is when you fill up the screen with falling powder. But moving solids is a good idea, just not for right now.