It would have been helpful to add a link to this story - I'll do it for you.
Just a couple of quotes from article which is opposite what you claim in the OP. Also, it isn't going to happen overnight if it does as Tom has already said.
" But other analysts say that because American and US Airways overlap on just 12 routes—nearly all between their hubs—the merger wouldn't significantly increase fares."
"Jonathan Kletzel, head of the transportation practice at accounting and consulting firm PricewaterhouseCoopers LLP, said mergers don't necessarily lead to an increase in fares, and that from 2004 through 2011—a period that includes some of the biggest airline mergers—domestic airfares actually decreased by nearly 1% a year when adjusted for inflation."
"The recent wave of consolidation has meant higher profits and more stability for the industry, which has led airlines to invest in technology, new airplanes and better customer service. "It's not all bad news," said Mr. Seaney of FareCompare. "A healthy airline industry means a better flying experience overall.""