Er, "mainstream media" includes local news organizations...
When I grew up, local news was (surprise!) somewhat independent. Each television show had their own particular style of reporting or emphasis on what they tended to cover. I never considered local TV news to be terribly great in quality compared to the newspapers, but at least it wasn't monotone in nature.
So, there's nothing really wrong with the spiel per se (everyone has bias and opinions). Except that it was pushed to 100s of television stations via local news channel, to be read exactly out loud as written.
That's a pretty strong argument to torpedo the merger. Forget the political angle... apparently on Sinclair stations, there's a push to make local news and national news no different. This is what really is terrible. Approve the merger and you probably will get the same effect of the massive consolidation of US radio in the 1990s after the Telecommunications Act of 1996 (the mergers pretty much killed off programming that focused on local and regional tastes or music, killed off the already dying freeform DJ outside of community radio, which stagnated music variety considerably (https://www.pbs.org/wgbh/pages/frontline/shows/music/perfect...)...)
In the end, what Sinclair is doing is probably going to is not good for their long term business anyways. This mass-scripted thing has drawn quite a bit of negative publicity, for a start. And using the 1990s radio example, well, terrestrial radio is quickly dying due to irrelevance. The "800 lb gorilla" winner of the 1996 Telecommunications Act, Clear Channel (iHeartMedia), recently went bankrupt after all.