I've been using reddit for 12 years and the quality of the site has gone down considerably in just the last few years. The trolling, bots, and hate speech seem to be rampant. There has been a concerted effort for certain ideological groups to take over popular subreddits like country and city specific subreddits to push a specific agenda.

All this happens while plenty of other niche subreddits go on as usual. I'm subscribed to a bunch of subreddits with great communities.

While popularity certainly has a huge role in the downfall, Reddit was already Internet-popular years ago. I think the problem now is that Reddit is being gamed pretty hard and they don't have the means to fix it. And it's not just a technological problem, it's a social one.