1. For media to serve the public interest, they need to be controlled, which ultimately means funded, by the public.
2. Intermediaries such as advertisers, corporations, government agencies or quasi-public corporations like CPB end up influencing media organizations more than the public who funds them.
3. Pure public funding--subscription-funded journalism, for example--doesn't seem to be able to support even current newsrooms
4. I don't trust politicians and the government to decide which journalists get subsidized and which do not, nor to create a set of rules (which would almost certainly run afoul of the First Amendment).
The media postal subsidy ingeniously navigated this thicket. Maybe something else can too.