How are you going to code it and make it work?
If a block of sand higher than 1 layer is surrounded by air, every n tick the air will fill with 1 layer of sand and the original sand layers will decrease in size.
As the result may be an impossible division ( eg a layers of 2 surrounded by 2 blocks of air, each block should countain 2/3 cube meters of sand), the calculation will be rounded randomly , so we'll end up with a whole layer thickness.
Will it go through blocks with gaps, even if they are visual?
I did not think about it, but that would be a great thing if sand was able to go through things, maybe even boots and armor (in that case it would be a kind of slowness debuff) ; and it would definitely increase the map's badassness (I'm already thinking of an empty room with only a thin string/fillet I don't know how to say this flowing from the ceiling)
Lag. Lots of lag. Minecraft already tends to lag a bit on PC.
I'm not a professionnal of game physics, so if I was a developper I would add this only as a gamerule. (but I'm pretty sure that the 8-layers system explained above won't be that much a problem, meanwhile I won't force players to use a feature making them laggy )