Mainstream music is going to sell more, regardless of whether it's "good" or not. As Wes pointed out, the quality of music is mostly subjective. There are mathematical reasons as to why a collection of notes is consonant or dissonant, and almost every person will agree that dissonant sounds are horrendous, but the quality of a properly written song (meaning that it follows the mathematical rules of chord progression, consonance, etc.) is going to be subjective.
With that being said, the obvious way to make money with a radio station is to play mainstream music. "Good" or not, it sells, and it brings your station popularity, even if it's only popularity with teens. This isn't what happened with 100.5.
100.5 turned into a talk radio station that has to compete with NPR, JOX and other talk radio stations that are already in the area. Is this better than filling a niche market for indie music? Maybe, I don't know; I don't have the figures for Birmingham talk radio stations. However, what I do know is that they turned a music station into a talk radio station, and that seems like silly management to me.
I don't know many people who listen to talk radio. Of those who do, they either listen to sports radio, or they're older and they listen to the news. Within the city of Birmingham, 66.1% of the population is 44 or under. Only 13.5% are 65 or older. The median age is 34.
Based upon my experiences, most older people who listen to talk radio do so for the news or political commentary, both of which they probably get from the television moreso than the radio. Afterall, most people don't listen to radio in their homes. If you're retired and are at home, in all likelihood you're going to be watching the television.
I can see why 100.5 stopped playing indie music. However, I just don't see the reason why they decided to go to talk radio. It would make more sense to me if they had switched to mainstream, classic rock, rock, country, rap or some other genre of music. I honestly don't see how talk radio is going to make them more money than a music station would.