For one thing, the article talks about antique furniture costing thousands or tens of thousands of dollars. Selling that to a crowd of debt-saddled Millennials on the basis of "social justice" is pretentious to the extreme. After all, my organic latte is already resting atop my bamboo yoga mat... so I've done my part.
More seriously though, I've always assumed that the antiques market is driven by nostalgia and longing for the past. The younger generation today seems to have very low levels of nostalgia and longing for the past. Specifically FOR "social justice" reasons.
Please don't get me wrong, I'm not making some absurd Tumblr argument that antique chairs are racist or anything. I'm just noting that less nostalgia for the past probably lowers the demand for antiques, and surely that plays a role alongside the supply rising due to Baby Boomer retirements.