Yes we will. Planet killer sized asteroids are the easiest to detect and track and are also the least common. City killers are harder: there is a chance we won't one of those coming and there are many more of them out there.
Asteroids that hit Earth are in near-Earth orbits. For a first intercept event the odds of an asteroid just happening to hit an Earth sized target as opposed to, say, a 1000x Earth sized "near miss" target is incredibly unlikely. We get a few near-miss events of a given asteroid before an impact happens.
The asteroid that wiped out the dinosaurs was between 3 and 10 miles diameter. 2014 UN271 is 100km-370km.
Would we have discovered 2014 UN271 earlier had its orbit brought it closer to Earth? It’s making its closest approach in 2031 (ok, 11AU away!), only 17 years after discovery.
And sure, incredibly unlikely something that size would hit Earth, but just an example of something that is very large, and discovered shortly before it makes its closest approach.
Objects with highly eccentric orbits are both very uncommon and very unlikely to intercept Earth. The period in which they have near misses is on the order of thousands of years at least. Even if we could see them from their high altitude we don't have enough information to predict where they'll be on the next intercept.
My understanding is that this kind of threat is usually lumped in with comets, whereas threat asteroids are lumped in with low eccentricity big rocks.
Asteroids that hit Earth are in near-Earth orbits. For a first intercept event the odds of an asteroid just happening to hit an Earth sized target as opposed to, say, a 1000x Earth sized "near miss" target is incredibly unlikely. We get a few near-miss events of a given asteroid before an impact happens.