I think it's more that Reddit grew so much that the original community gets drowned out by everyone else. You can't have a community that big without it becoming filled with, for lack of a better word, normies.

What narrative supports The_Donald and the numerous left leaning subreddits both being there? Honestly I hear this a lot and am curious. I believe it is because they are focused on retention numbers which thrive off focused insular communities to be clear.

Extreme polarization is good for page views.

I’m not sure that is actually what is happening overall, but the narrative that you hear a lot (and that would explain fairly intolerant subreddits like T_D and worldnews) is about filter bubbles and more generally the idea that we now have the ability to ignore other people’s dissent, and become complacent of more one-sided, prejudiced ways of presenting information. To take examples outside of reddit: it’s fairly easy to watch several hours of television every day, entirely about news, think that you know very well what is happening and be completely flabbergasted that anyone (reasonable) supports President Trump or reciprocally, that anyone (reasonable) doesn’t. This feels fairly new, not entirely driven by internet (more by the increase in news sources, i.e. likely the drop in production costs, and ad technology).

I suspect that this is happening more than before, but that the opposite (people finding structured, intelligent opposing viewpoints) is also happening more --people spend a lot more time informing themselves overall-- but that the more constructive phenomena is less destructive therefore harder to notice.