Competition with MasterCard, Discover, and Amex should hold them back from doing this, because at such punitive rates merchants would definitely stop accepting them.
Some of the Amex fee changes will likely have been as a result of a nudge from the EU ruling that includes their double-branded cards (e.g. British Airways AmEx) in the 0.3% fee cap in Europe which previously only affected visa and MasterCard.
I believe Amex is only able to get away with its highest-in-the-industry rates because of the association of Amex cards with business expense accounts and high income earners. This is changing now (https://www.ft.com/content/715d785e-23fc-11e8-ae48-60d3531b7...) especially after years of competition from banks like Chase, who have been working on creating what is essentially a structural clone of American Express’ network (https://www.reuters.com/article/us-jpmorganchase-creditcards...).
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