It's very hard to kill a two-sided market when it gets established. Look at Craigslist or Twitter.
An interesting thing about Reddit to me is many people who look at it see strongly offputting things like being overrun by image memes but when you look at it closely there is so much there about so many subjects. From time to time you find those brilliant social media posts that remind me of some of the emails from the Enron Emails data set where people talk about what they did for training in the army. Perhaps that is what keeps people coming back or maybe they are all about the memes.
The fact that people often append "reddit" to their Google queries is a testament to this. Even if they aren't active participants in these niche communities, they know that they are the easiest place to find reasonably reliable information from other humans on them.
If these types of communities stop flourishing on Reddit for any reason, then the site will become much easier to replace by any other generic meme factory.
They considered this around the 3rd party app fiasco and it was clear old Reddit has a few years left because mods allhate the new Reddit UI
I've seen no evidence that anything they've introduced in the last 10 years has contributed to any of that growth, and my evidence is that just about every feature announced or implemented since then is now either gone or largely ignored by the userbase.