I disagree. I do think you're right in the short term. But those that were already listening had to start listening sometime. They weren't always conservative news-talk listeners; they migrated there as their tastes changed. Sometimes that's with age, but certainly not always. Any potential future listeners have to first sample the product, and the only way that can happen now is for the station to be on FM. Maybe they come for traffic or weather, but as their tastes change they find themselves tuning in for the format. Please don't take this to mean that every 18-24 listener will eventually turn 65 and automatically tune to news-talk. Everybody is different. But a format that attracts the older members of a population has to get those listeners from somewhere. For those potential new listeners, today, they are too young to even find an AM station. To them, AM is their grandparent's radio.
Sure, you're right that switching to FM isn't going to be a magic pill that doubles the audience. But without the switch the format's audience will eventually die.