Addendum to my post about the Sears Tower: Speaking of a "world government" that can take the place of "all humanity" in preserving a building (or site), one thing I neglected to mention was the UNESCO World Heritage Convention of 1972.
This is an approximation of the sort of overall governing body that could represent "humanity", but it has a basic problem: The sites remain the responsibility of the state powers that own them. (I guess this assumes that any site worthy of the list must be state-owned, which is a particularly UN-oriented idea.) The UN itself has no responsibilities at all, nor does it have the power to actually preserve the sites, other than the threat of delisting them.
UNESCO's role is essentially that of organizer, not really preserver. Being toothless, it can't enforce preservation; lacking ownership, it also assume no risks. By contrast, a city historical society is typically more interventionist, since it actually has the power to block building modifications and sales, while assuming none of the risks of ownership.
Interestingly, the example I used in my previous post - Notre Dame de Paris - is not a World Heritage Site, but Notre Dame de Reims is.