Although I disagree with foggyb larger view and I think Moore's law (or more correctly we are interested in Koomey's law as it relates to encryption) is good for at least three or four decades and possibly a century he likely is right about war.
Since WWII at least in the US (and I would imagine around the world) military tactics have changed. The goal is no longer to secure territory, land, strategic points ("take the hill") it is to utterly dominate the enemy and destroy both their ability to wage war (kill troops instead of taking land) and their ability to finance the war.
WWII defenses and offenses were fairly matched. B-52 bombers for example could level a factory but they often missed and routinely bombers would be destroyed enroute. Since WWII the destructive capacity of offensive weapons has improved by magnitudes but defensive systems haven't.
In a modern global war of full spectrum dominance you simply couldn't defend your industrial assets. Stealth bombers, high speed drones, ground hugging cruise missiles, bunker buster ordinances, ballistic missiles, long range pinpoint accurate field artillery, etc would rapidly overwhelm any defensive systems.
How many cruise missiles can a Intel FAB take before it is a $20B pile of rubble? How many stealth bomber runs can a nuclear power plant take before it is molten radioactive slag and there is no power to run your $20B Silicon chip FAB?
The most effective way to "win" is destroy the enemies ability to wage war so both (all?) sides will.
The good news is all the nations capable of waging such a war have nuclear weapons and unstoppable delivery systems. Any such war would inevitably escalate to nuclear force. Either pre-emptively to "end the war before it starts" or defensively as one side starts to lose and sees nuclear weapons as the only way to regain a fighting chance. Nuclear escalation wouldn't stop (when 1 million of your citizens die as a leader you will strike back with similar force). Limited "strategic exchanges" would escalate to "counterforce" (google it) strikes and eventually full scale "countervalue" strikes.
So yes when 90%+ of human race is wiped out and technological progress is pushed backwards 200 or so years as humans cling to a shattered and poisoned planet then foggy is likely right Moore's law won't continue and your encrypted file is likely safe.
Still now that we are WAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAYYYYYYYYYYYYYYYYYYYYYY off topic.
Hoping for a nuclear war (or any other improbable scenario) to keep your encrypted secrets safe really doesn't have much value. It would be like not getting fire insurance, counting on the fact that the day your house catches fire it will be raining ... hard.
Any estimate for the long term strength of a cipher should be based on
the most plausible and likely scenario. That scenario is that
Moore's law (or more accurately Koomey's law) will continue for next 30+ years. So if your secret must remain protected even 30 year from now you should assume computers will eventually have 1 million times as much performance per watt (30 years at doubling ever 18 months). When choosing a cipher strength that should be your target.
Now if your assumption is WRONG well most likely it is wrong on the short side (nuclear war, lack of demand for faster chips, technological brick wall) and your file is still safe. On the other hand if your assume Moore's law won't hold and it does well you are fracked.
It is all about being conservative.