You need to do some research. In 1990 there were 4,987 AM stations in the USA. As of late 2009: 4,790. By late 2013: 4,728. I don't have 2014 totals, I've been told to expect at least another 14 deletions.
Given the combined realities of AM core audience reaching mortality and the one viable mainstream talk format; sports slowly migrating to FM, it is fair to expect the declines to accelerate.
LCG
There are far more viable AM formats than talk. Sports has only migrated in some markets, leaving many on AM. Religious options, of which there are many, both non-profit and commercial, are very viable as are ethnic formats in many markets. And in the smaller markets, AMs can still do community based programming with success.
Also note that quite a few of the station deletions in the last 15 years have been buy-to-shut-down transactions where turning off an AM allows another to improve its signal or coverage.
Of the rest, some have been stations that had to move and could not make their directional work at a new location or could not find a location. Others were stations that should never have existed anyway.
The FCC's current hold on new stations means that there are no stations replacing the numbers of stations lost.