1: you lack the alloys needed to make something tough enough to resist everything apart from Nukes. the world has plenty of armour piercing and precision destruction weapons to deal with anything you can come up with
2: you lack a place large enough to house such a massive construction
3: you lack the control of physics that would allow you to use something like Mechagodzilla effectively
4: a single building won't cause a depression.
5: to construct something of that size you'll need alot of workers. it only requires 1 spy to expose it all to me, or the CIA or other Secret Service agencies, and Intelligence Agencies around the globe, including MI6, MI5 and so on
6: i'd already have control over all weapons storages around the globe [i may even have a weapons manufacturing branch of my own by that time], so even if you managed to construct a Mechagodzilla, you wouldn't be able to arm it with the kind of high grade, powerful weapons available to the military and my organisation
7: you need blueprints of Mechagodzilla [which, btw, doesn't exist, therefore making this impossible to acquire] to be able to get your construction anywhere...
so yeah. another failure...
