In order to issue co-branded cards for direct competitors, American Express (and Chase and others) will need to have separate teams working on each brand's products. I don't think there is any reason to expect Hilton to mimic Marriott's change.
And even if they did, there was an actual announcement months in advance that the $300 Marriott credit was changing. Can’t see why they wouldn’t do that for the $250 airline credit, and oh look, we’re less than a month and a half from getting a new credit since it resets in January 1.