No permanence is required. Dollar cost averaging makes sense only when two necessary conditions are met: (1) one is not good at calling tops or bottoms (in individual stocks or indices,) or is not advised by such a person, and (2) the trend over your investment time horizon is up.
"history says it will recover"
I agree it will recover. But that is not the issue. Saying that it will recover is nothing more than a statement that the country will not vanish off the face of the earth. The issue is: will it recover over most people's investment horizon?
History actually gives us very few independent data points on which to base faith in the vaunted "long term." Ten to twenty, at most. Not enough on which to bet the farm.
Strangely, people's confidence in trends over various timescales is inversely proportional to the amount of available data.
And I guess it's how you, as an individual, perceive things going.
You wrote:
(1) one is not good at calling tops or bottoms (in individual stocks or indices,) or is not advised by such a person
My grandfather, who I didn't have the opportunity to meet, was a margin officer for one of the major brokerages in Chicago in the 20s and 30s. In those days it was a position of some repute, because he was essentially deciding who the brokerage would lend money to, often using nothing more than a hunch.
My father told of his father sleeping in the office during the darkest days of the crash of 1929, and had a memorable anecdote about advice on stocks and why he never gave it.
It seems my grandfather was reasonably successful with his own investments, and was often asked for investment advice.
"I never will give it', my father quoted him as saying. "Because if I'm right they'll say, 'I told him what I wanted and he bought it for me', and if I'm wrong, it'll be 'that SOB lost all my money'"
My father didn't give investment advice either.