It does.
When a really nice niche subreddit is thrust into the lime-light by becoming a default, it usually dies from an influx of low-quality default users. If it doesn't die from that, the moderation team has to really step up their job - a job they're not getting paid to do which means moderation teams get overworked and burned out, and the quit and the sub dies from lack of moderation. If the subreddit is survives that, the more successful approach seems to be for moderation to become very heavy-handed, which does quite well in a number of places, but also forces a change to the subreddit's culture that was doing well before it became a default.
If the answer is to not become a default sub, well, then the low-quality of the default has affected a good quality sub from becoming a default and prevented quality content from reaching more users.
Unfortunately, a good moderation team doesn't just appear, and one way of acquiring talent is to gasp pay for it. Reddit's avoidance of paying moderators results in relying on unpaid volunteers to do the bulk of the work, so good teams are few and far between, and horrible racist troll-y shit is de jure.
Reddit wants to be "the front page of the internet", but when the default not-logged-in front page features shit like "What's the difference between a feminist and a gun? (self.Jokes)" (pulled just now), it's never going to rise above being dismissed as the front page for horrible trolls, racists, and misogynists.
If I'm supposed to make an account then wade through a pile of shit subreddits just to find an AskHistorians diamond or two, how about I just don't wade through the pile of shit at all?