Most brutalism was never intended to last. It was intended to be a quick/cheap answer to get people acceptable housing in the cities. Then they would build something nicer for people to live in as the economy gets richer. Which is why it so often is associated with decay these days - the structure still stands, but it has outlasted the expected lifespan.

There are burtalism structures that were intended to be beautiful and last. They do that well (well beauty is in the eye of the beholder), but the majority was quick and cheap above all else.