And you also have to remember that in the past, only a few individuals had information and they were portrait as righteous, reliable and leaders. After the banalization and propagation of books and the implantation of the giant web of comunication that was instaled in the world (the internet), the acquirement of knowledge became easier.. So easy that now, everyone is the owner of the truth. But, to acquire the "truth", the person needs a reliable source (newscast, newspapers, books, lectures made by masters of such subject) and the person, who was raised in a specific type of community will opt for the type of truth that fits his/hers cosmovision.
Now, for a healthy discussion, I opt for the "facts against facts", for exemple, if you have a different point of view than me and you have facts that can support your ideals, that is totally valid, if you have facts, regardless of the main source you use (it can be left, right or even impartial (which I think that all new sources should strive to be) to base your argument, and I too, then we can have a decent conversation. You said quite rightly, "rivals in arguments" will often say that the source of facts that the other one use is biased, well, for that I can only say that people should be more reasonable, I mean, there are biased news ("fake news"), but some few just report the news. Even with a complete impartiality, the opposition will ignore or not acknowledge it as a valid news source.
The fact you used as a exemple is perfect, both sides have decent arguments to support them. Now, if you want to defend a point of view without a reliable source of facts or even with a biased news source, how do you want me to expand the conversation ? Sure, you believe in what you are saying, but... I can't trust you and by that, I can't discuss the topic with you, so I just ignore.
unfortunately, most people go with the easy way and only perceive what they read/hear as truth/right, they don't compare it to their moral standards, if they have one to begin. Congratulations, now we have "maneuver mass"*, people that do what they are told and believe what they hear, without a single point of judment or inner questionament of the veracity of such fact. What I notice is that there are times where people agree in a common point but suddenly, they divide themselves in rival groups. So much that being moderate or being central about a topic in these days is impossible.
*(I hate to use that term BTW, but... There is no better term to describe this type of people).