No Way
The KBOs we have detected are hundreds of km not m in diameter. The K/T impactor was only 6 to 10 km in diameter. Most comets are smaller still yet plenty big enough to cause large scale havoc. We can not detect these objetcs until they get to about Jupiter distance and develope a coma because they are so dark in a fully frozen state. Given their long periods we will most likely detect a comet impactor when it is headed for us without enough warning to prevent impact (6 mo to 2 yrs). Humanity's only hope in such a case is even more powerful rockets than we have now and better yet eggs in another basket. You don't get either by abandoning a manned space program.
Regards,
Mark