A lot of the default sub-reddits are weakly moderated, and so very vulnerable to brigading, bullying, harassment, and boundary pushing. You see a lot of carefully crafted racism and "anti-sjw" posts in the default sub-reddits, for example. Bit by bit, drop by drop, they apply a consistent pressure. The mods of those sub-reddits are too dumb to figure out what's going on, or just don't care. Unfortunately, all of this slowly drives away people who are sensitive enough to care, or just ordinary people. For them, it's not worth it to "fight the good fight", is some dumb sub-reddit worth that cost? Of course not, so they leave and go somewhere else. This is a slow dynamics problem, and the "good" parts of reddit aren't strong enough to save the site.
Realistically, if reddit itself does nothing then it's maybe a 50/50 chance whether or not these forces eventually poison the site for average users, and people just stop going there. For most people these things aren't super noticeable, yet, because they're still not the dominant aspect of the site, but they've driven away who knows how many people, and continue to do so, while the racists, homophobes, and misogynists still have their little clubs on the site. Some of the worst sub-reddits were banned a while back, but many horrid ones still exist. And the people who spend their time there spend their time posting and voting on other parts of reddit as well.