You're kidding right? A 400 meter diameter asteroid impact would make those events seem trivial by any destructive standards.
So what? Does that mean everybody is going to commit suicide and rob banks, rape and pillage?
13000 died this weekend in tsunamis. With warning of even a few hours few would have died. With warning of a few weeks no one would have died.
With warning of a few weeks we could mount massive computer simulations to determine a fairly prcise point of impact and do mass evacuations.
People can handle this stuff if they are simply told about it. The idea that we need government to hide the truth from us is frankly rather insulting to me, but if that's what gets you thru the night, you're welcome to it.