Those 15% rates were there because a storm had already hit. And because Paul Volcker was a realist who wanted to put the country back on a solid footing.
In 1979 inflation was still boiling, but after a few years as disinflation took hold under Volcker, anyone who bought long term treasuries at those high rates was set for a very nice retirement.
Many people did not see these long term treasuries as gifts from heaven, they thought that inflation would continue up forever. They wanted gold, T-bills, famland etc.