You're correct in many ways. However...
1. Detection network will be VERY expensive to achieve and maintain for (say) a thousand years. How do we do it? Who pays?
2. Prediction of the course of bodies detected far enough off is almost certainly going to be subject to "chaotic" motion problems. Determination of mass/velocity/position as well as mass/velocity/position of all significant gravitational influences with precision sufficient is going to be very expensive (both time and money), and by necessity in most cases, the modelling of the path is going to result in a range of probability of striking Earth.
3. Who decides (for the next thousand years) what constitutes a threat we will respond to and which do they determine to ignore? What is the benefit or cost of correct calls?
4. Who designs and pays for the "mass drivers" or whatever other technology is used. Who decides how many of these devices to send up in the "swarm"? Surely, it will be to reduce the probability to some amount. Who determines that probability?
Got the idea? Though it may be a wonderful thought experiment, designing a way to deal with these events seems well beyond our ability right now... Of course, if you were dictator, and had a well defined and certain line of succession for the next millenium, you might be able to pull it off!
Who pays for the system of detecting and tracking North Atlantic icebergs? The longer it is maintained, the cheaper it will be to maintain it, as further assets come on line.
2. Prediction of the course of bodies detected far enough off is almost certainly going to be subject to "chaotic" motion problems.
True, just as air traffic control is somewhat chaotic and costly. However, automated analysis programs can highlight suspected objects, and decisions can be made at the appropriate time, depending on the threat level assessed.
3. Who decides (for the next thousand years) what constitutes a threat we will respond to and which do they determine to ignore? What is the benefit or cost of correct calls?
Essentially, what would be needed is a semi-military force, similar in function to our Coast Guard, with clear guidelines for how to assess threat levels in a timely fashion designed to provide ample time to take action.
Needless to say, the benefit of correct calls is the survival of billions of human beings.
4. Who designs and pays for the "mass drivers" or whatever other technology is used.
Whoever is far-sighted and responsible enough to realize there is a threat to be responded to. Bear in mind, while we have been discussing the avoidance of having asteroids target Earth, that is not the only possibility. It may be advisable to have a Home System Defense Network to prevent cranky and obstreperous pranksters from sending multi-megatons down as a playful greeting to an old girlfriend.
"... designing a way to deal with these events seems well beyond our ability right now..."
Nothing I have discussed has been even remotely difficult, except for a certain level of anticipated scale of activity. The mass-drivers could easily be assembled from essentially off the shelf components.
The easy way to address all your concerns is to open the space portals to economic enterprise. Then it would be a simple matter to task the investors in space activity with the burden and responsibility of policing their surroundings. There is but one obstacle to that. The 1967 Outer Space Treaty prevents profit-seeking activities in space. The United States should withdraw from that treaty, and allow investors to follow their dreams.
It will be to the benefit, and possible survival, of all of us.