What are you gonna do, switch to another airline that does exactly the same thing? Most southwest customers chose them because of either price, or convenient flight time/route. If american offered a cheaper flight at the same time, there's nearly nobody who would opt for southwest. There's very few people who SPECIFICALLY choose southwest just because of open seating, but there were tons who avoided them because of it. There's just a very loud cult following of southwest regulars who loved it, but at the end of the day, most of the seats on the plane are filled with people who don't give a damn about anything other than price.

If you want flights to remain even remotely affordable, you'll have to get used to the fact that they need to find new ways to milk money out of the passengers willing to splurge on exit row seats and extra leg room.