Even if one had near indefinite computing power, making such a calculation would be extremely difficult simply due to the lack of precision of observations. Small changes in the estimated size or trajectory of an asteroid could have drastic results on their long-term behavior. Comets are even more ill-behaved. The limiting factor to such calculations would be at least as much observational as calculational.

Moreover, since large impacts are extremely rare threats, dealing with individual impact threats as they arise is a much more optimal strategy.