Now, the cost is approximately <5 minutes per flashcard to give you roughly 95% recall (more details: http://www.gwern.net/Spaced%20repetition#what-to-add ); so the question is, does not knowing something you could've put into a flashcard - the consequences of not knowing it, the time it takes to look it up, whether you will even know to look it up, etc - outweighs the cost of a few minutes' review over that time period.
I think for a lot of stuff this can be true: shell scripting is not going away in 20 years, for example. If you're programming in it routinely, a language you will use for only 10 years can be worth memorizing for a while. If you're using a tool every day for the next year, there's going to be a lot worth memorizing there too.
> In programming, there are very few things that will still
> be the same in 20+ years and we have no idea what those
> things are.
SRS is well designed for gradually changing bodies of knowledge. New cards are added and come up more often, older cards can be retired but they rarely come up anyway.