I agree with much of what you say, but I’m not so quick to label all media as having a political agenda. Certainly there are a number of media that exist to serve a political ideology and are quite up-front about how they lean (ThinkProgress is one), and there are plenty of media on both ends of the political spectrum that fit this description.
However, the larger media — the for-profit ones owned by ginormous corporate interests — are less motivated by political ideology than by the god they bow down to — RATINGS. And Trump is giving them material to keep people tuned in. In spades.
Keeping this in mind, Trump’s obvious attempt to limit access to him to just the fawning media isn’t going to work for him. This alone is going to be reported ad nauseum by all the news departments on major networks, the major cable news channels (even Fox News), and the major newspaper dailies. That we have never had a President make this blatant of an attempt to control press access to him is a major news story in itself. And it doesn’t put Trump in a favorable light.
Furthermore, shutting certain media out of press briefings isn’t going to prevent them from reporting on Trump or his staff. They’ll just use other sources to get their information. If Trump wants to declare war on the media, he certainly can, but it will be a war he will lose.