It's a "perceived" loss of value.
Some of us got a letter enticing us to sign up for a Amex Plat (sometimes co-branded) by explaining that although the fee was large, it was paid off easily through use and exploitation of benefits. So, for example, there was a line item in this bit of marketing material that estimated the cost of a return flight on QF and showed that this, along with a free flight from the points you would earn and the travel insurance, added up to more than the card fee, meaning that you would be getting your money's worth with Amex Plat.
People may not actively detest the change for that reason but they may decide to use that as a means of negotiating a better deal out of Amex (as people will do for all credit cards, i.e. pick on something then threaten to cancel the card to try and get something else).