The physics of tomorrow though will be the physics of today however: a many months flight time out to an orbital object to affect trajectory change is a response time so far away as to be meaningless: any opponent with an ICBM strike capability on Earth could happily blast away your entire civilization, and reasonably expect to use the same technology that enables that ICBM capability to blast away (or deflect) your asteroid.
Put it another way: for considerably less expenditure of effort, you could just build a hydrogen-bomb large enough to obliterate Earth's biosphere completely[1]
Consider that ICBM's might not work forever. If e.g. new types of anti-ballistic missile defenses were to become cheap and reliable, then ICBM/SLBM's could even become obsolete (say, within the century). If that happened, there'd be a slew of new arms races, which are unimaginable to us, being beyond our technology horizon.
Physics isn't changing over time, but engineering is.
