[Alpha]

Megastructures of any kind are expansive. And there is always a safety problem. How good will a megastructure stand strong storms and earthquakes? What about sabotage or terrorism?

Where to evacuate the people when a structure like "X-seed 4000" was damaged so, that it may collapse?

I think one will only built such things when there is no other way.

The best construction site for megastructures would not be earth. It will be space, either in form of large space stations, space elevators, huge solar power stations, solar shields, scientific instruments, etc..
A megastructure in space could be much easilier built, because it has not to stand winds or earthquakes and in many cases no very large forces.