Thanks, that makes sense. Actually, it wasn't "topsoil", but that $4/
bag mix. Maybe "planting mix" instead of "potting mix"? Again, it
appears mostly organic stuff because it's fluffy when it comes out of
the bag, and for some weeks/months after using it. Would be a good
trick to take plain topsoil and make it look that way. Good point
about sand migrating downward in the pot. Had not thought of that.
Also, the shrinkage, which I figured would be an inconvenience, does
indeed have an advantage in removal from the pot.

I'm thinking peat moss is good for fluffing it up as well, but does
that break down rapidly enough that you'd end up with the same organic
glue at the end of a season?