Earlier quoted context omitted.
We already ran out of microns in the 1990's.
Silicon atoms are only 0.235nm apart. So at 1nm you only have 5 atoms left. At 0.2nm you have one atom lleft and it stops behaving like a semiconductor. So there are real physical limits to nm scale devices that don't apply to micron sized ones. Admittedly were not as close to those limits as it seems since "nm" quoted on CPUs etc is no longer a literal size of the object. But we're getting there...
nm process numbers are based on the theoretical feature size that a planar transistor process would need to be equivalent to the used process. Since this number is purely theoretical the lower bound is zero, it can go well below the plank length.