Well, you are entitled to your opinion,Why?
Those not interested in points will stay.
Those interested in points will find other alternatives will earn less points.
The turnover part may drop from those manufacturing points. If the enablers can find ways to reduce their fees that may not occur.
Personally I suspect Amex will have run the numbers and know that for many that they will still earn more points than their competitors. VISA/MC earn has been slashed and so in this competitive world Amex did not need to be as generous as they once were.
Amex may lose some, but offsetting this their cost in buying points from airlines will plummet.
There is no doubt that benefits of amex will reduce significantly,
Some may stil think its the best earning card out which i dont disgaree with, but my personal belief is that overall members, new sign ups and turnover will decrease noticeably as to how much, i have no idea
My best case scenario for amex is to keep both cards with a reduction of 50% spend,whilst the worst will be cancelling of all cards.
However, like you said, i may not represent the typical amex customer