I remember seeing someone post a softfork that allowed to issue more than 21 million bitcoins, so clearly any sort of thing is possible via softfork/hardfork.

Since a hardfork attack (or softfork) can always be attempted, it seems the only defense against something that is wrong is for there to be an outcry about it.

P.S. We can avoid the extreme N*N sig tx attack without breaking any existing tx by setting the limit to allow 1MB tx, but that still avoids problems from larger blocks