Asteroid mining will not bring the resources back to Earth. They will be used for fueling exploration/colonization further out.

The bigger threat to Gold on Earth is the fact that there is close to half a quadrillion in gold in our oceans. Right now it is a challenge since it is parts per trillion so there is no economical way to pull the gold out.

Give it some time and I would bet that is not the case.

Demographics also come into play. Millennials do not look at gold like Boomers do.