I think you are partially correct. It's not that citizens are becoming less interested over time. What is decreasing over time is the ***popularity of the idea*** that people will spend considerable time researching news online to become fully informed. This idea is turning out not to be true. People don't like to spend a lot of time reading different news sources. They might read only what's personally interesting, or they might just believe news that is biased for their party affiliation. (For example, Republican voters might watch only Fox News, and Democrats might watch only CNN.) Or they might support party candidates without knowing much about their positions. They are compensating for lack of facts by just supporting party candidates. (Not very well-written, in my opinion.)

When the author is describing the "related alternative," he is talking about different opinions on how citizens consume news. The main view is the idea that citizens are informed, meaning they will scour the news and make informed decisions (the idea that is not popular anymore). An alternative to this opinion is the idea that citizens only read what's important to them personally and otherwise follow cues like party affiliation (e.g., Democrat vs. Republican). A second alternative is the opinion that citizens do respond to news, but only when it's alarmist.

I must say again that I think this paragraph was very poorly written. I think what the author might have ***meant*** to say is that there are three types of citizens: 1) the informed citizen; 2) the partisan citizen; and 3) the citizen who only responds to alarmist news.