Earlier quoted context omitted.
Forgive my ignorance, but what would they be - the complete implosion of all forms of known security, or something else? This is a bit beyond my ken :)
Among other things, mostly encryption. Most of our current methods depends on P != NP. So no need for 0 days if you can just read at encrypted data as if it wasn't.
Not really. They depend on guessing being slow. P, just P, can do slow. P can be bigger than the universe even with low values of n.