Why was Seward's purchase of Alaska viewed as a "folly" by many Americans? What changed their minds?
It was viewed as a frozen wasteland in the north and of no particular value.
Seward bought Alaska from the Russians in 1867. Immediately following the Civil War, America went on an expansion spree. Their eye focused first on Oklahoma and other plains states which were still territories. Although it was never said aloud, American politicians likely felt that one day they would annex Canada and that buying Alaska would simply round that out. But the citizens of 1867 saw no real value in colonizing Alaska. All fishing and crabbing could be done on the west coast up to Washington. And so the called it "Seward's Folly" thinking he had just thrown away $7.2 million.
But in 1899 when gold was discovered in the Klondike that all changed of course.