Until enough small instances fail/disappear. It’s kind of like with users, if you have a bunch of users that only comment twice a year..if one leaves, meh not a big deal. As more and more of them leave that is 2^n less comments you are getting. At first the drain isn’t noticeable but by the time it is it’s likely too late, too many people are gone and your website is a shell of what it used to be.
People keep saying to push people into a bunch of smaller servers but the same thing will happen. Especially since the person that started the server will be likely footing the bill from their own income. If they lose interest or don’t feel like paying for it anymore, poof there goes their server and users. (Yes their users can make new accounts on new instances but how many times do you think people are going to put up with losing their account)