While most frequent fliers in the US are stuck with one airline due to the presence of fortress hubs, there are a few cities that are competitive. In Los Angeles, for example, the four major carriers (UA, AA, DL and WN) are pretty much neck and neck, with each having about 15-20% market share. Chicago (AA and UA) and New York are other examples.

The way I see it, these few competitive cities are what is stopping the airlines from devaluing their programs. If American slashes benefits and raises redemption prices, for example, then people in Dallas and Miami will have no choice but to put up with it, but people in LA and Chicago will migrate to other carriers.

As the three megacarriers shrink the fortress hubs they inherited in mergers (CVG, MEM, potentially CLE and PHX) and Southwest expands, it actually reduces the number of people tied down to one carrier and makes it harder for airlines to devalue their programs without a backlash.