I don't think it's fair to call that "scales quadratic". Certainly the energy stored in the total number of blocks is growing quadratic by digging deeper, but i

I don't think it's fair to call that "scales quadratic". Certainly the energy stored in the total number of blocks is growing quadratic by digging deeper, but it also ignores that costs and effort grow when digging deeper. In effect, adding a block at the bottom comes with substantially higher effort than adding a block right next to it on the same level.