One way we know what news is is seeing where the news story comes from. For example, when I want to look for news about the Los Angeles Rams, I go to the Twitter pages of Rams reporters such as Rich Hammond and Vincent Bonsignore. Those two men are reporters for the Rams and have access to the team so I trust them more so than someone at ESPN who may just be watching the games but not interacting with the team aside from that.
For political news it seems that people go to the big news companies such as Fox and CNN and big news companies are trusted sources for news by millions of people. Having a respected brand in terms of trust is even more important than ever nowadays due to readers paying closer attention to fact checking than ever before.
We need journalists because they can get information that the average Joe cannot. I could not walk in to the Rams practice facility and get an interview with Sean McVay, but somebody like Rich or Vincent could because they are members of the press who cover the Rams.
Advantages of the third model of gate keeping are that more opinions will be shared and more conversation will be generated about topics of interest. However, some disadvantages are there will definitely be more bias and with that people will be less trustworthy of news since the potential for bias will be bigger than it is now.