Just because something is 1. an asset, 2. 'alternative', if it doesn't generate an income then the value is just supply and demand set against a negative cost of carry.
I would put collectable cars, art, wine, antiques, not commercially rented assets like real estate, boats and planes in that bucket.
Perhaps its also demographics, maybe if when you retire at 55/60/65 buying one of these cars or other collectable items seems cool, but at 65/70/75 you then want to sell and baby boomers are now in the process of selling.
Next up the suburban McMansions.