Second, your statement #2 is true, it caused a mass extinction event and probably paved the way for humanity to evolve. If we however get hit by an asteroid of that size again, chances are high that most of the population will die out.
There are not an infinite amount of asteroids, and in large time frames you can reduce their numbers. Therefore, we have a large hit once a century. Large, but largely inconsequential (The Earth is empty. Try this: Pick random coordinates and look them up on a map. Repeat. How long before you end up w/ 10 km of an inhabited area?)
Theories are infinite, funds are not. This is an ineffective solution (we can only deflect small and well predicted orbits) to an unlikely problem: mass extinction from asteroids hasn't happened in a very long time (millions of years) -> Bayesian inference -> very low probability of occurring soon. It will happen. But in a millions years
Compare that with the plethora of reasons we might die out in the next 10, 20, or 100 years.
Mass extinctions, from your number, appear to be a once in a 132 Mya event. [0]
Humans have been on Earth 0.1 Mya, 0.2Mya; statistically we'll be around for a total of 0.2 to 0.4 Mya in total. [0][1]
Therefore, there is a 1-330 chance of the two time lines overlapping.
[0] Assuming the present time is random, I believe the best estimate of event frequency is that we're in the middle of two occurrences; hence the doubling. [1]I'm being generous here. I don't think we have 0.2 Mya left in us, primarily because for the first time in history we have the ability to kill ourselves off. Therefore the assumption that out present in at a random position of the human experience used in [0] is invalid. Since we probably only have 100 years left, that's a 1-1E6 possibility facing us.
I think the theory is deflection of this object from its orbit could have all sorts of unintended consequence like a domino effect that cause either a chain reaction of events that lead this object or others on a collision course with Earth.