Lifespan and timespan
I reckon that if future generations DO obtain regular life expectancies of a century or two, then they will also adopt a more forward-looking and cautious approach to chance accidents. They will WANT to deal with wandering asteroids.
Without being a total head-in-the-sand Ostrich, it seems that it doesn't matter too much if we do nothing till 2020 (but a tagging mission would make a lot of sense at that date)