A very fast concrete/glass/metal/plastic[1]-eating nanobug, programmed to start from the top-down, disposing of its digestion products in some sort of safe way[2], completely avoids consuming anything biological, or bedrock[3] and leaves the people unharmed. Well, with some other minor inconveniences (water flowing in, biological waste unhindered, possibly a whole lot of people needing new fillings (as well as eyeglasses) if they didn't keep their mouths shut during the deconstruction phase...)
But, well... Minor (ahem, NPI!) teething problems aside,
it might just work!!!No, I can't see
any problems with this. I'm not sure why they haven't already done this, in fact.
[1] Remember to wear all natural fibres on Nanobug day! But feel free to advise anyone else you're
not totally repulsed b,y already, that Polyester Is In This Year
TM!
[2] That's the biggy. What do you send into the atmosphere, what else do you allow to pool? You could probably fudge the "arches fall once the keystone is eaten!" problem by having the 'consumption foam' be supporting enough of everything it's still in the process of eating that it holds up
most structures.
[3] Or just have it range-limited by some form of external radio reference, so that you end up with a flat depression into the landscape, at worst.