The thing with these sites is that only a tiny proportion of users create content. For Reddit it’s even worse, since they also need subreddit moderators, essentially employees working for free. So, tens of thousands of users leaving will definitely have an impact since in this case it’s old power users who make content, and mods. Will the site “die”? Nope, we’ll see it working for a long, long time, but will be a shadow of what it used to be, for example i remember the Obama AMA and many similar high-profile ones, that’s not gonna happen anymore or will be reduced greatly.