I see this as a calculated half-full threat. AA won't eliminate the change fee for inflexible tickets, but may well eliminate SDC. That would leave the standard domestic change fee for the typical inflexible ticket at $200 and not drop it to $75 or $0 on the day of travel. People who require flexibility will purchase flexible fares and the revenue from change fees is protected.
The reality is that UA & DL will move in lockstep and that will leave WN in a pickle. It cannot survive taking over broad swaths of business which changes "for free" constantly or it will suffer the same problems others suffer now. Or, its fare structure, not exactly a bargain today, will take a significant jump.