because all remnants of the construction that were not an essential part of the complex were cleaned up. If they had not been these conversations would not have to take place.
Here is your math
at 50 metres 80% of the Volume was in place.
If one allows for a hive like make up of solid interspersed with sand and marl filled cavity,
thus reducing the overall piece count to closer to 1.5 million..
one only has to allow for a 100,000 or less pieces in the upper levels
at 100metres 97% of the volume is in place
20+ years. 24 hours a day ( more of the heavier work at night)
add one heck of a lot of organisation
clear easily followed templates and instructions
and a well fed and nurtured dedicated work force
no problem, to my thinking. And I don't have to prove any negatives