Whether we knew/know about an asteroid strike doesn't change whether the strike will occur. As such, the actual likelihood of an asteroid hit is the same, either way. What has changed is our knowledge that it is going to occur.
Put differently, there are a finite number of asteroids in the solar system. If one othem is on a trajectory that will eventually impact the earth, the likelihood of an earth impact is unchanged whether we know it or not. Likewise, if none of them are on a trajectory to impact earth, the likelihood of an earth impact is unchanged whether we know it or not.
At this point in time, there are only two options - either the earth will be hit or it will never be hit. The more we know about the asteroids and their trajectories does not change those results (unless by knowing, we have a means to divert the collision, which currently, we can't).