The assumption that small businesses would see a drop in marketing efficiency might assume that advertising costs on Facebook remain constant. In a perfect world, I would think that a reduction in ROAS on Facebook would precipitate a decrease in ad costs, as businesses would be less willing to pay a premium for Facebook ads.
In the long term, I think Facebook is the biggest loser in this scenario, not the small businesses.
In the long term, I think Facebook is the biggest loser in this scenario, not the small businesses.