This is a rather facile answer to an important question and one that can be easily gleaned from the Op-Ed pages of The Guardian. You fail to mention the ownership structure of the media and how this impacts its editorial decisions, its access to resources and the proliferation of unpaid internships and insecure labour practices that exclude most people but the independently wealthy from entering the profession.
With regards to your third point. Since when has disinformation been the sole purview of government funded media? The ongoing campaign by the Murdoch consortium against peer-reviewed science in the field of climate change is surely the most obvious example of disinformation and a highly successful one at that. Furthermore, with regards to your second point on false balance, you failed to bring up the most salient issue of climate change or international relations. In the latter, for example, it is common practice on TV news to present two "analysts" that agree in principal on a subject but share a difference in methodology. To illustrate, that will be a discussion on what should be the most appropriate approach to dealing with the Syrian civil war with "regime change" in Syria being one position or "just" a no-fly zone the other without the position of those opposed to military solutions (let alone the Syrian government) being presented.
That certain media outlets have found such receptive audiences in the West is because, in addition to instances of disinformation, they cover stories that are excluded from the mainstream media discourses due to the editorial decisions at newspapers and TV news that has much to do with the personal biases of management as it is the influence of a concentrated ownership structure. To return to the example of climate change once more this is an issue that should be front page news in all outlets yet it is consistently regulated to back pages in favour of political gossip of no consequence or fails to be mentioned in articles about the economy. Further, it came as an embarrassment to outlets like MSNBC and NYT that it was RT America that extensively covered the Occupy Movement when it emerged when, for weeks, it was practically ignored thus enabling this government-owned outlet to gain influence and audience share in the US.
There are legitimate concerns about the "Fourth Estate" and I think one should trust the layman's observation that in the West, especially in the English and Spanish speaking worlds, it is utterly dysfunctional in terms of its theoretical role of keeping democracies honest. Your first point about "increasing partisanship" overlooks the rich history of conflict between the "newspaper barons" and the labour movement in the UK and US whereby the newspapers were often seen as crucial areas of political contest, which belies the notion that partisan news is some kind of novel phenomenon. Indeed, this idea that polarisation is an exception and not the norm is merely part of liberal political theory that is true in some circumstances but usually not in most. The decline of the free press in Italy in the 20s and Wiemar Germany are instructive because it was the fascists that argued that partisanship in the media was damaging to the "national ethos" even though their newspapers were no less partisan than those of their opponents.
Perhaps, what we are witnessing in the West is a wising up among Westerners from the fog of Cold War era propaganda that their news media institutions were honest brokers of the Truth untainted by material self-interest or partisanship. To use a quote attributed to Zdeněk Urbánek:
"In dictatorships we are more fortunate than you in the West in one respect. We believe nothing of what we read in newspapers and nothing of what we watch on television, because we know it's propaganda and lies. Unlike you in the West, we've learned to look behind the propaganda and to read between the lines, and unlike you, we know that the real truth is always subversive."
To answer this question, we must first establish whether or not people's trust in the media is actually in decline. Luckily, this is quite easy to do, since polling agencies in many countries keep track of public trust in assorted governmental and non-governmental institutions. What we find is quite interesting: yes, levels of trust in the mainstream media are dropping rapidly, but only in Western democracies (for examples, see the US, the Netherlands, Spain and Australia). In Asia and Latin America, a recent study concludes, people tend to trust their televisions, newspapers and online media more and more as time progresses. A preliminary theory that we can infer from this raw data is that trust in media is declining in places with high levels of press freedom and online media consumption, but stays the same or increases in places where press freedom is increasing, staying constant, or edging downwards from a sub-obtimal level. We can easily get lost trying to find explanations for these trends in economic development, the rise of technology in developing countries, or the effects of new media on authoritarian governments, but it's probably best to explore these questions some other time.
So, if we only look at Western democracies, why are so many people losing trust in an institution that has so long been a hallmark of democratic societies? In my view, there are three main contributing factors, which I will treat individually below.
First, the commercial free market increasingly incentivises partisanship. The media landscape today is dotted with websites, TV programmes and radio shows with an explicitly partisan bent. Much of this can be traced back to the more general issue of political polarisation present in much of the Western world. This pattern is very clear in the US (think of Fox News, MSNBC, Rush Limbaugh or the Daily Show), but similar developments can be seen elsewhere as well. Polarised media consumption habits mean that a consumer is more likely to only trust what they themselves consume, and to draw a clear distinction between 'trusted' and 'biased' media. This leaves less and less demand for outlets that give a voice to both sides of a debate. Combine this with (often justifiable) accusations of false balance being aimed at journalists who try to be non-partisan, and you end up with a media landscape with little room for compromise.
Second, newspaper advertisement revenues are in steep decline. Traditional news outlets, especially newspapers, have moved online in pursuit of the money river. This poses two problems for general levels of trust in the mainstream media; on the one hand, professional media outlets have less resources available to do their work properly, leading to budget cuts and a precarious job market for trained journalists. On the other hand, moving online means competing with people with no formal training in journalism, but with the same access to consumer markets, advertisement revenue and distribution methods as everyone else. This dual development has led to an online media landscape where consumers are subjected to less and less professional journalistic output. In combination with the need for output to go viral (gather up a lot of clicks and likes), this means that fact-checked, well-researched content is disincentivised, leading, again, to increasingly partisan consumer behaviour.
The third factor I want to point out is disinformation. The last decade or so has seen the rise of several media outlets, owned by governments, that put forward 'alternative' viewpoints supposedly ignored by the mainstream media. While there is nothing wrong in principle with establishing an alternative TV channel or website in a country that enjoys freedom of the press (or anywhere, really), it would be a mistake to assume that this is a government's only goal. By consistently putting forward alternative theories to unfolding events, framing issues as being deliberately ignored or wrongly covered by other media, and presenting any given topic in a gloomy (geo)political context, the goal is not to convince audiences that the mainstream media is wrong and alternative media is right. Rather, such outlets seek to sow the seeds of doubt, to create a postmodern consumer base that sees the truth as mostly subjective and assumes the media are usually lying to them. While we can certainly debate the effectiveness of such disinformation efforts, it remains true that governmental efforts to 'influence public opinion abroad' through the media are affecting our trust in the journalistic field.