The percentage of listener support varies from station to station. The membership money is the basis for the government money. So there is a motivation to attract membership money, and interact with that membership. The programming at these stations is built around attracting membership. Once that's established, then they look at filling in the budget gaps with local business underwriters. I'm not aware of a public non-com station that's changed programming because of local business. It begins with what the membership wants. In the markets I watch, music programming has been replaced with more news and talk because of declining listener support for the music programming.
Back to alternative, if you look at non-commercial AAA stations such as WXPN or WUMB or KEXP, they are all fiercely supportive of their music formats. They aren't going to flip to news or some other format because of underwriting. On the other hand, they expect a dedicated membership base that financially supports their music format. Alternative fans think they can get what they want for free. That's not going to happen. Alternative fans have to organize and coalesce around something in the genre that they all agree on. Right now, that appears to be classics. Once we get to current music, the audience splits into a bunch of smaller sub-groups, and that's not good for funding any kind of radio station.