[Al-Khwarizmi]

If the thesis was true, we'd expect rich people who will never be compelled to move against their will, or to move into less space, would prefer cluttered homey interiors, and poor people would prefer sparse & modern. In reality, the biggest boosters of modern decor are rich people.

Are you sure? Maybe some of them are, but it's also quite typical of rich people to live in a refurbished 19th century home with ornate mouldings, antique furniture, bookshelves, rugs, paintings, and a large amount of carefully curated "clutter". While working-class people almost universally move towards minimalism when they renovate, with the rich it's much more divided.